diff --git a/README.md b/README.md
index 01afccb..dfb67c6 100644
--- a/README.md
+++ b/README.md
@@ -18,7 +18,9 @@
     1.  [Coding style](#coding-style)
     2.  [Submitting patches](#submitting-patches)
 
-3.  [Bug reports](#bug-reports)
+3.  [PHP implementation](#php-implementation)
+
+4.  [Bug reports](#bug-reports)
 
 ## How to use {#how-to-use}
 
@@ -74,6 +76,15 @@
 If you would like to contribute to **libavifinfo**, please follow the steps for
 **libaom** at https://aomedia.googlesource.com/aom/#submitting-patches.
 
+## PHP implementation
+
+`libavifinfo` was [implemented](https://github.com/php/php-src/pull/7711) into
+**php-src** natively and is available through `getimagesize()` at head. If it is
+not available in the PHP release version you use, you can fallback to
+`avifinfo.php` instead.
+
+See `avifinfo_test.php` for a usage example.
+
 ## Bug reports {#bug-reports}
 
 Bug reports can be filed in the Alliance for Open Media
diff --git a/avifinfo.php b/avifinfo.php
new file mode 100644
index 0000000..d46be59
--- /dev/null
+++ b/avifinfo.php
@@ -0,0 +1,783 @@
+<?php
+/**
+ * Copyright (c) 2021, Alliance for Open Media. All rights reserved
+ *
+ * This source code is subject to the terms of the BSD 2 Clause License and
+ * the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
+ * was not distributed with this source code in the LICENSE file, you can
+ * obtain it at www.aomedia.org/license/software. If the Alliance for Open
+ * Media Patent License 1.0 was not distributed with this source code in the
+ * PATENTS file, you can obtain it at www.aomedia.org/license/patent.
+ */
+
+namespace Avifinfo;
+
+const FOUND     = 0; // Input correctly parsed and information retrieved.
+const NOT_FOUND = 1; // Input correctly parsed but information is missing or elsewhere.
+const TRUNCATED = 2; // Input correctly parsed until missing bytes to continue.
+const ABORTED   = 3; // Input correctly parsed until stopped to avoid timeout or crash.
+const INVALID   = 4; // Input incorrectly parsed.
+
+const MAX_SIZE     = 4294967295; // Unlikely to be insufficient to parse AVIF headers.
+const MAX_VALUE    = 255;        // Be reasonable. Avoid timeouts and out-of-memory.
+const MAX_TILES    = 16;         // Be reasonable.
+const MAX_PROPS    = 32;         // Be reasonable.
+const MAX_FEATURES = 8;          // Be reasonable.
+const UNDEFINED    = 0;          // Value was not yet parsed.
+
+/**
+ * Reads an unsigned integer with most significant bits first.
+ *
+ * @param binary string $input     Must be at least $num_bytes-long.
+ * @param int           $num_bytes Number of parsed bytes.
+ * @return int                     Value.
+ */
+function read_big_endian( $input, $num_bytes ) {
+  if ( $num_bytes == 1 ) {
+    return unpack( 'C', $input ) [1];
+  } else if ( $num_bytes == 2 ) {
+    return unpack( 'n', $input ) [1];
+  } else if ( $num_bytes == 3 ) {
+    $bytes = unpack( 'C3', $input );
+    return ( $bytes[1] << 16 ) | ( $bytes[2] << 8 ) | $bytes[3];
+  } else { // $num_bytes is 4
+    // This might fail to read unsigned values >= 2^31 on 32-bit systems.
+    // See https://www.php.net/manual/en/function.unpack.php#106041
+    return unpack( 'N', $input ) [1];
+  }
+}
+
+/**
+ * Reads bytes and advances the stream position by the same count.
+ *
+ * @param stream               $handle    Bytes will be read from this resource.
+ * @param int                  $num_bytes Number of bytes read. Must be greater than 0.
+ * @return binary string|false            The raw bytes or false on failure.
+ */
+function read( $handle, $num_bytes ) {
+  $data = fread( $handle, $num_bytes );
+  return ( $data !== false && strlen( $data ) >= $num_bytes ) ? $data : false;
+}
+
+/**
+ * Advances the stream position by the given offset.
+ *
+ * @param stream $handle    Bytes will be skipped from this resource.
+ * @param int    $num_bytes Number of skipped bytes. Can be 0.
+ * @return bool             True on success or false on failure.
+ */
+// Skips 'num_bytes' from the 'stream'. 'num_bytes' can be zero.
+function skip( $handle, $num_bytes ) {
+  return ( fseek( $handle, $num_bytes, SEEK_CUR ) == 0 );
+}
+
+//------------------------------------------------------------------------------
+// Features are parsed into temporary property associations.
+
+class Tile { // Tile item id <-> parent item id associations.
+  public $tile_item_id;
+  public $parent_item_id;
+}
+
+class Prop { // Property index <-> item id associations.
+  public $property_index;
+  public $item_id;
+}
+
+class Dim_Prop { // Property <-> features associations.
+  public $property_index;
+  public $width;
+  public $height;
+}
+
+class Chan_Prop { // Property <-> features associations.
+  public $property_index;
+  public $bit_depth;
+  public $num_channels;
+}
+
+class Features {
+  public $has_primary_item = false; // True if "pitm" was parsed.
+  public $has_alpha = false; // True if an alpha "auxC" was parsed.
+  public $primary_item_id;
+  public $primary_item_features = array( // Deduced from the data below.
+    'width'        => UNDEFINED, // In number of pixels.
+    'height'       => UNDEFINED, // Ignores mirror and rotation.
+    'bit_depth'    => UNDEFINED, // Likely 8, 10 or 12 bits per channel per pixel.
+    'num_channels' => UNDEFINED  // Likely 1, 2, 3 or 4 channels:
+                                          //   (1 monochrome or 3 colors) + (0 or 1 alpha)
+  );
+
+  public $tiles = array(); // Tile[]
+  public $props = array(); // Prop[]
+  public $dim_props = array(); // Dim_Prop[]
+  public $chan_props = array(); // Chan_Prop[]
+
+  /**
+   * Binds the width, height, bit depth and number of channels from stored internal features.
+   *
+   * @param int     $target_item_id Id of the item whose features will be bound.
+   * @param int     $tile_depth     Maximum recursion to search within tile-parent relations.
+   * @return Status                 FOUND on success or NOT_FOUND on failure.
+   */
+  private function get_item_features( $target_item_id, $tile_depth ) {
+    foreach ( $this->props as $prop ) {
+      if ( $prop->item_id != $target_item_id ) {
+        continue;
+      }
+
+      // Retrieve the width and height of the primary item if not already done.
+      if ( $target_item_id == $this->primary_item_id &&
+           ( $this->primary_item_features['width'] == UNDEFINED ||
+             $this->primary_item_features['height'] == UNDEFINED ) ) {
+        foreach ( $this->dim_props as $dim_prop ) {
+          if ( $dim_prop->property_index != $prop->property_index ) {
+            continue;
+          }
+          $this->primary_item_features['width']  = $dim_prop->width;
+          $this->primary_item_features['height'] = $dim_prop->height;
+          if ( $this->primary_item_features['bit_depth'] != UNDEFINED &&
+               $this->primary_item_features['num_channels'] != UNDEFINED ) {
+            return FOUND;
+          }
+          break;
+        }
+      }
+      // Retrieve the bit depth and number of channels of the target item if not
+      // already done.
+      if ( $this->primary_item_features['bit_depth'] == UNDEFINED ||
+           $this->primary_item_features['num_channels'] == UNDEFINED ) {
+        foreach ( $this->chan_props as $chan_prop ) {
+          if ( $chan_prop->property_index != $prop->property_index ) {
+            continue;
+          }
+          $this->primary_item_features['bit_depth']    = $chan_prop->bit_depth;
+          $this->primary_item_features['num_channels'] = $chan_prop->num_channels;
+          if ( $this->primary_item_features['width'] != UNDEFINED &&
+              $this->primary_item_features['height'] != UNDEFINED ) {
+            return FOUND;
+          }
+          break;
+        }
+      }
+    }
+
+    // Check for the bit_depth and num_channels in a tile if not yet found.
+    if ( $tile_depth < 3 ) {
+      foreach ( $this->tiles as $tile ) {
+        if ( $tile->parent_item_id != $target_item_id ) {
+          continue;
+        }
+        $status = get_item_features( $tile->tile_item_id, $tile_depth + 1 );
+        if ( $status != NOT_FOUND ) {
+          return $status;
+        }
+      }
+    }
+    return NOT_FOUND;
+  }
+
+  /**
+   * Finds the width, height, bit depth and number of channels of the primary item.
+   *
+   * @return Status FOUND on success or NOT_FOUND on failure.
+   */
+  public function get_primary_item_features() {
+    // Nothing to do without the primary item ID.
+    if ( !$this->has_primary_item ) {
+      return NOT_FOUND;
+    }
+    // Early exit.
+    if ( empty( $this->dim_props ) || empty( $this->chan_props ) ) {
+      return NOT_FOUND;
+    }
+    $status = $this->get_item_features( $this->primary_item_id, /*tile_depth=*/ 0 );
+    if ( $status != FOUND ) {
+      return $status;
+    }
+
+    // "auxC" is parsed before the "ipma" properties so it is known now, if any.
+    if ( $this->has_alpha ) {
+      ++$this->primary_item_features['num_channels'];
+    }
+    return FOUND;
+  }
+}
+
+//------------------------------------------------------------------------------
+
+class Box {
+  public $size; // In bytes.
+  public $type; // Four characters.
+  public $version; // 0 or actual version if this is a full box.
+  public $flags; // 0 or actual value if this is a full box.
+  public $content_size; // 'size' minus the header size.
+
+  /**
+   * Reads the box header.
+   *
+   * @param stream  $handle              The resource the header will be parsed from.
+   * @param int     $num_parsed_boxes    The total number of parsed boxes. Prevents timeouts.
+   * @param int     $num_remaining_bytes The number of bytes that should be available from the resource.
+   * @return Status                      FOUND on success or an error on failure.
+   */
+  public function parse( $handle, &$num_parsed_boxes, $num_remaining_bytes = MAX_SIZE ) {
+    // See ISO/IEC 14496-12:2012(E) 4.2
+    $header_size = 8; // box 32b size + 32b type (at least)
+    if ( $header_size > $num_remaining_bytes ) {
+      return INVALID;
+    }
+    if ( !( $data = read( $handle, 8 ) ) ) {
+      return TRUNCATED;
+    }
+    $this->size = read_big_endian( $data, 4 );
+    $this->type = substr( $data, 4, 4 );
+    // 'box->size==1' means 64-bit size should be read after the box type.
+    // 'box->size==0' means this box extends to all remaining bytes.
+    if ( $this->size == 1 ) {
+      $header_size += 8;
+      if ( $header_size > $num_remaining_bytes ) {
+        return INVALID;
+      }
+      if ( !( $data = read( $handle, 8 ) ) ) {
+        return TRUNCATED;
+      }
+      // Stop the parsing if any box has a size greater than 4GB.
+      if ( read_big_endian( $data, 4 ) != 0 ) {
+        return ABORTED;
+      }
+      // Read the 32 least-significant bits.
+      $this->size = read_big_endian( substr( $data, 4, 4 ), 4 );
+    } else if ( $this->size == 0 ) {
+      $this->size = $num_remaining_bytes;
+    }
+    if ( $this->size < $header_size ) {
+      return INVALID;
+    }
+    if ( $this->size > $num_remaining_bytes ) {
+      return INVALID;
+    }
+
+    $has_fullbox_header = $this->type == 'meta' || $this->type == 'pitm' ||
+                          $this->type == 'ipma' || $this->type == 'ispe' ||
+                          $this->type == 'pixi' || $this->type == 'iref' ||
+                          $this->type == 'auxC';
+    if ( $has_fullbox_header ) {
+      $header_size += 4;
+    }
+    if ( $this->size < $header_size ) {
+      return INVALID;
+    }
+    $this->content_size = $this->size - $header_size;
+    // Avoid timeouts. The maximum number of parsed boxes is arbitrary.
+    ++$num_parsed_boxes;
+    if ( $num_parsed_boxes >= 4096 ) {
+      return ABORTED;
+    }
+
+    $this->version = 0;
+    $this->flags   = 0;
+    if ( $has_fullbox_header ) {
+      if ( !( $data = read( $handle, 4 ) ) ) {
+        return TRUNCATED;
+      }
+      $this->version = read_big_endian( $data, 1 );
+      $this->flags   = read_big_endian( substr( $data, 1, 3 ), 3 );
+      // See AV1 Image File Format (AVIF) 8.1
+      // at https://aomediacodec.github.io/av1-avif/#avif-boxes (available when
+      // https://github.com/AOMediaCodec/av1-avif/pull/170 is merged).
+      $is_parsable   = true;
+      if ( $this->type == 'meta' ) {
+        $is_parsable = ( $this->version <= 0 );
+      } else if ( $this->type == 'pitm' ) {
+        $is_parsable = ( $this->version <= 1 );
+      } else if ( $this->type == 'ipma' ) {
+        $is_parsable = ( $this->version <= 1 );
+      } else if ( $this->type == 'ispe' ) {
+        $is_parsable = ( $this->version <= 0 );
+      } else if ( $this->type == 'pixi' ) {
+        $is_parsable = ( $this->version <= 0 );
+      } else if ( $this->type == 'iref' ) {
+        $is_parsable = ( $this->version <= 1 );
+      } else if ( $this->type == 'auxC' ) {
+        $is_parsable = ( $this->version <= 0 );
+      }
+      // Instead of considering this file as invalid, skip unparsable boxes.
+      if ( !$is_parsable ) {
+        $this->type = 'unknownversion';
+      }
+    }
+    // print_r( $this ); // Uncomment to print all boxes.
+    return FOUND;
+  }
+}
+
+//------------------------------------------------------------------------------
+
+class Parser {
+  private $handle; // Input stream.
+  private $num_parsed_boxes = 0;
+  private $data_was_skipped = false;
+  public $features;
+
+  function __construct( $handle ) {
+    $this->handle   = $handle;
+    $this->features = new Features();
+  }
+
+  /**
+   * Parses an "ipco" box.
+   *
+   * "ispe" is used for width and height, "pixi" and "av1C" are used for bit depth
+   * and number of channels, and "auxC" is used for alpha.
+   *
+   * @param stream  $handle              The resource the box will be parsed from.
+   * @param int     $num_remaining_bytes The number of bytes that should be available from the resource.
+   * @return Status                      FOUND on success or an error on failure.
+   */
+  private function parse_ipco( $num_remaining_bytes ) {
+    $box_index = 1; // 1-based index. Used for iterating over properties.
+    do {
+      $box    = new Box();
+      $status = $box->parse( $this->handle, $this->num_parsed_boxes, $num_remaining_bytes );
+      if ( $status != FOUND ) {
+        return $status;
+      }
+
+      if ( $box->type == 'ispe' ) {
+        // See ISO/IEC 23008-12:2017(E) 6.5.3.2
+        if ( $box->content_size < 8 ) {
+          return INVALID;
+        }
+        if ( !( $data = read( $this->handle, 8 ) ) ) {
+          return TRUNCATED;
+        }
+        $width  = read_big_endian( substr( $data, 0, 4 ), 4 );
+        $height = read_big_endian( substr( $data, 4, 4 ), 4 );
+        if ( $width == 0 || $height == 0 ) {
+          return INVALID;
+        }
+        if ( count( $this->features->dim_props ) <= MAX_FEATURES &&
+             $box_index <= MAX_VALUE ) {
+          $dim_prop_count = count( $this->features->dim_props );
+          $this->features->dim_props[$dim_prop_count]                 = new Dim_Prop();
+          $this->features->dim_props[$dim_prop_count]->property_index = $box_index;
+          $this->features->dim_props[$dim_prop_count]->width          = $width;
+          $this->features->dim_props[$dim_prop_count]->height         = $height;
+        } else {
+          $this->data_was_skipped = true;
+        }
+        if ( !skip( $this->handle, $box->content_size - 8 ) ) {
+          return TRUNCATED;
+        }
+      } else if ( $box->type == 'pixi' ) {
+        // See ISO/IEC 23008-12:2017(E) 6.5.6.2
+        if ( $box->content_size < 1 ) {
+          return INVALID;
+        }
+        if ( !( $data = read( $this->handle, 1 ) ) ) {
+          return TRUNCATED;
+        }
+        $num_channels = read_big_endian( $data, 1 );
+        if ( $num_channels < 1 ) {
+          return INVALID;
+        }
+        if ( $box->content_size < 1 + $num_channels ) {
+          return INVALID;
+        }
+        if ( !( $data = read( $this->handle, 1 ) ) ) {
+          return TRUNCATED;
+        }
+        $bit_depth = read_big_endian( $data, 1 );
+        if ( $bit_depth < 1 ) {
+          return INVALID;
+        }
+        for ( $i = 1; $i < $num_channels; ++$i ) {
+          if ( !( $data = read( $this->handle, 1 ) ) ) {
+            return TRUNCATED;
+          }
+          // Bit depth should be the same for all channels.
+          if ( read_big_endian( $data, 1 ) != $bit_depth ) {
+            return INVALID;
+          }
+          if ( $i > 32 ) {
+            return ABORTED; // Be reasonable.
+          }
+        }
+        if ( count( $this->features->chan_props ) <= MAX_FEATURES &&
+             $box_index <= MAX_VALUE && $bit_depth <= MAX_VALUE &&
+             $num_channels <= MAX_VALUE ) {
+          $chan_prop_count = count( $this->features->chan_props );
+          $this->features->chan_props[$chan_prop_count]                 = new Chan_Prop();
+          $this->features->chan_props[$chan_prop_count]->property_index = $box_index;
+          $this->features->chan_props[$chan_prop_count]->bit_depth      = $bit_depth;
+          $this->features->chan_props[$chan_prop_count]->num_channels   = $num_channels;
+        } else {
+          $this->data_was_skipped = true;
+        }
+        if ( !skip( $this->handle, $box->content_size - ( 1 + $num_channels ) ) ) {
+          return TRUNCATED;
+        }
+      } else if ( $box->type == 'av1C' ) {
+        // See AV1 Codec ISO Media File Format Binding 2.3.1
+        // at https://aomediacodec.github.io/av1-isobmff/#av1c
+        // Only parse the necessary third byte. Assume that the others are valid.
+        if ( $box->content_size < 3 ) {
+          return INVALID;
+        }
+        if ( !( $data = read( $this->handle, 3 ) ) ) {
+          return TRUNCATED;
+        }
+        $byte          = read_big_endian( substr( $data, 2, 1 ), 1 );
+        $high_bitdepth = ( $byte & 0x40 ) != 0;
+        $twelve_bit    = ( $byte & 0x20 ) != 0;
+        $monochrome    = ( $byte & 0x10 ) != 0;
+        if ( $twelve_bit && !$high_bitdepth ) {
+            return INVALID;
+        }
+        if ( count( $this->features->chan_props ) <= MAX_FEATURES &&
+             $box_index <= MAX_VALUE ) {
+          $chan_prop_count = count( $this->features->chan_props );
+          $this->features->chan_props[$chan_prop_count]                 = new Chan_Prop();
+          $this->features->chan_props[$chan_prop_count]->property_index = $box_index;
+          $this->features->chan_props[$chan_prop_count]->bit_depth      =
+              $high_bitdepth ? $twelve_bit ? 12 : 10 : 8;
+          $this->features->chan_props[$chan_prop_count]->num_channels   = $monochrome ? 1 : 3;
+        } else {
+          $this->data_was_skipped = true;
+        }
+        if ( !skip( $this->handle, $box->content_size - 3 ) ) {
+          return TRUNCATED;
+        }
+      } else if ( $box->type == 'auxC' ) {
+        // See AV1 Image File Format (AVIF) 4
+        // at https://aomediacodec.github.io/av1-avif/#auxiliary-images
+        $kAlphaStr       = "urn:mpeg:mpegB:cicp:systems:auxiliary:alpha";
+        $kAlphaStrLength = 44; // Includes terminating character.
+        if ( $box->content_size >= $kAlphaStrLength ) {
+          if ( !( $data = read( $this->handle, $kAlphaStrLength ) ) ) {
+            return TRUNCATED;
+          }
+          if ( substr( $data, 0, $kAlphaStrLength ) == $kAlphaStr ) {
+            // Note: It is unlikely but it is possible that this alpha plane does
+            //       not belong to the primary item or a tile. Ignore this issue.
+            $this->features->has_alpha = true;
+          }
+          if ( !skip( $this->handle, $box->content_size - $kAlphaStrLength ) ) {
+            return TRUNCATED;
+          }
+        } else {
+          if ( !skip( $this->handle, $box->content_size ) ) {
+            return TRUNCATED;
+          }
+        }
+      } else {
+        if ( !skip( $this->handle, $box->content_size ) ) {
+          return TRUNCATED;
+        }
+      }
+      ++$box_index;
+      $num_remaining_bytes -= $box->size;
+    } while ( $num_remaining_bytes > 0 );
+    return NOT_FOUND;
+  }
+
+  /**
+   * Parses an "iprp" box.
+   *
+   * The "ipco" box contain the properties which are linked to items by the "ipma" box.
+   *
+   * @param stream  $handle              The resource the box will be parsed from.
+   * @param int     $num_remaining_bytes The number of bytes that should be available from the resource.
+   * @return Status                      FOUND on success or an error on failure.
+   */
+  private function parse_iprp( $num_remaining_bytes ) {
+    do {
+      $box    = new Box();
+      $status = $box->parse( $this->handle, $this->num_parsed_boxes, $num_remaining_bytes );
+      if ( $status != FOUND ) {
+        return $status;
+      }
+
+      if ( $box->type == 'ipco' ) {
+        $status = $this->parse_ipco( $box->content_size );
+        if ( $status != NOT_FOUND ) {
+          return $status;
+        }
+      } else if ( $box->type == 'ipma' ) {
+        // See ISO/IEC 23008-12:2017(E) 9.3.2
+        $num_read_bytes = 4;
+        if ( $box->content_size < $num_read_bytes ) {
+          return INVALID;
+        }
+        if ( !( $data = read( $this->handle, $num_read_bytes ) ) ) {
+          return TRUNCATED;
+        }
+        $entry_count        = read_big_endian( $data, 4 );
+        $id_num_bytes       = ( $box->version < 1 ) ? 2 : 4;
+        $index_num_bytes    = ( $box->flags & 1 ) ? 2 : 1;
+        $essential_bit_mask = ( $box->flags & 1 ) ? 0x8000 : 0x80;
+
+        for ( $entry = 0; $entry < $entry_count; ++$entry ) {
+          if ( $entry >= MAX_PROPS ||
+               count( $this->features->props ) >= MAX_PROPS ) {
+            $this->data_was_skipped = true;
+            break;
+          }
+          $num_read_bytes += $id_num_bytes + 1;
+          if ( $box->content_size < $num_read_bytes ) {
+            return INVALID;
+          }
+          if ( !( $data = read( $this->handle, $id_num_bytes + 1 ) ) ) {
+            return TRUNCATED;
+          }
+          $item_id           = read_big_endian(
+              substr( $data, 0, $id_num_bytes ), $id_num_bytes );
+          $association_count = read_big_endian(
+              substr( $data, $id_num_bytes, 1 ), 1 );
+
+          for ( $property = 0; $property < $association_count; ++$property ) {
+            if ( $property >= MAX_PROPS ||
+                 count( $this->features->props ) >= MAX_PROPS ) {
+              $this->data_was_skipped = true;
+              break;
+            }
+            $num_read_bytes += $index_num_bytes;
+            if ( $box->content_size < $num_read_bytes ) {
+              return INVALID;
+            }
+            if ( !( $data = read( $this->handle, $index_num_bytes ) ) ) {
+              return TRUNCATED;
+            }
+            $value          = read_big_endian( $data, $index_num_bytes );
+            // $essential = ($value & $essential_bit_mask);  // Unused.
+            $property_index = ( $value & ~$essential_bit_mask );
+            if ( $property_index <= MAX_VALUE && $item_id <= MAX_VALUE ) {
+              $prop_count = count( $this->features->props );
+              $this->features->props[$prop_count]                 = new Prop();
+              $this->features->props[$prop_count]->property_index = $property_index;
+              $this->features->props[$prop_count]->item_id        = $item_id;
+            } else {
+              $this->data_was_skipped = true;
+            }
+          }
+          if ( $property < $association_count ) {
+            break; // Do not read garbage.
+          }
+        }
+
+        // If all features are available now, do not look further.
+        $status = $this->features->get_primary_item_features();
+        if ( $status != NOT_FOUND ) {
+          return $status;
+        }
+
+        // Mostly if 'data_was_skipped'.
+        if ( !skip( $this->handle, $box->content_size - $num_read_bytes ) ) {
+          return TRUNCATED;
+        }
+      } else {
+        if ( !skip( $this->handle, $box->content_size ) ) {
+          return TRUNCATED;
+        }
+      }
+      $num_remaining_bytes -= $box->size;
+    } while ( $num_remaining_bytes > 0 );
+    return NOT_FOUND;
+  }
+
+  /**
+   * Parses an "iref" box.
+   *
+   * The "dimg" boxes contain links between tiles and their parent items, which
+   * can be used to infer bit depth and number of channels for the primary item
+   * when the latter does not have these properties.
+   *
+   * @param stream  $handle              The resource the box will be parsed from.
+   * @param int     $num_remaining_bytes The number of bytes that should be available from the resource.
+   * @return Status                      FOUND on success or an error on failure.
+   */
+  private function parse_iref( $num_remaining_bytes ) {
+    do {
+      $box    = new Box();
+      $status = $box->parse( $this->handle, $this->num_parsed_boxes, $num_remaining_bytes );
+      if ( $status != FOUND ) {
+        return $status;
+      }
+
+      if ( $box->type == 'dimg' ) {
+        // See ISO/IEC 14496-12:2015(E) 8.11.12.2
+        $num_bytes_per_id = ( $box->version == 0 ) ? 2 : 4;
+        $num_read_bytes   = $num_bytes_per_id + 2;
+        if ( $box->content_size < $num_read_bytes ) {
+          return INVALID;
+        }
+        if ( !( $data = read( $this->handle, $num_read_bytes ) ) ) {
+          return TRUNCATED;
+        }
+        $from_item_id    = read_big_endian( $data, $num_bytes_per_id );
+        $reference_count = read_big_endian( substr( $data, $num_bytes_per_id, 2 ), 2 );
+
+        for ( $i = 0; $i < $reference_count; ++$i ) {
+          if ( $i >= MAX_TILES ) {
+            $this->data_was_skipped = true;
+            break;
+          }
+          $num_read_bytes += $num_bytes_per_id;
+          if ( $box->content_size < $num_read_bytes ) {
+            return INVALID;
+          }
+          if ( !( $data = read( $this->handle, $num_bytes_per_id ) ) ) {
+            return TRUNCATED;
+          }
+          $to_item_id = read_big_endian( $data, $num_bytes_per_id );
+          $tile_count = count( $this->features->tiles );
+          if ( $from_item_id <= MAX_VALUE && $to_item_id <= MAX_VALUE &&
+               $tile_count < MAX_TILES ) {
+            $this->features->tiles[$tile_count]                 = new Tile();
+            $this->features->tiles[$tile_count]->tile_item_id   = $to_item_id;
+            $this->features->tiles[$tile_count]->parent_item_id = $from_item_id;
+          } else {
+            $this->data_was_skipped = true;
+          }
+        }
+
+        // If all features are available now, do not look further.
+        $status = $this->features->get_primary_item_features();
+        if ( $status != NOT_FOUND ) {
+          return $status;
+        }
+
+        // Mostly if 'data_was_skipped'.
+        if ( !skip( $this->handle, $box->content_size - $num_read_bytes ) ) {
+          return TRUNCATED;
+        }
+      } else {
+        if ( !skip( $this->handle, $box->content_size ) ) {
+          return TRUNCATED;
+        }
+      }
+      $num_remaining_bytes -= $box->size;
+    } while ( $num_remaining_bytes > 0 );
+    return NOT_FOUND;
+  }
+
+  /**
+   * Parses a "meta" box.
+   *
+   * It looks for the primary item ID in the "pitm" box and recurses into other boxes
+   * to find its features.
+   *
+   * @param stream  $handle              The resource the box will be parsed from.
+   * @param int     $num_remaining_bytes The number of bytes that should be available from the resource.
+   * @return Status                      FOUND on success or an error on failure.
+   */
+  private function parse_meta( $num_remaining_bytes ) {
+    do {
+      $box    = new Box();
+      $status = $box->parse( $this->handle, $this->num_parsed_boxes, $num_remaining_bytes );
+      if ( $status != FOUND ) {
+        return $status;
+      }
+
+      if ( $box->type == 'pitm' ) {
+        // See ISO/IEC 14496-12:2015(E) 8.11.4.2
+        $num_bytes_per_id = ( $box->version == 0 ) ? 2 : 4;
+        if ( $num_bytes_per_id > $num_remaining_bytes ) {
+          return INVALID;
+        }
+        if ( !( $data = read( $this->handle, $num_bytes_per_id ) ) ) {
+          return TRUNCATED;
+        }
+        $primary_item_id = read_big_endian( $data, $num_bytes_per_id );
+        if ( $primary_item_id > MAX_VALUE ) {
+          return ABORTED;
+        }
+        $this->features->has_primary_item = true;
+        $this->features->primary_item_id  = $primary_item_id;
+        if ( !skip( $this->handle, $box->content_size - $num_bytes_per_id ) ) {
+          return TRUNCATED;
+        }
+      } else if ( $box->type == 'iprp' ) {
+        $status = $this->parse_iprp( $box->content_size );
+        if ( $status != NOT_FOUND ) {
+          return $status;
+        }
+      } else if ( $box->type == 'iref' ) {
+        $status = $this->parse_iref( $box->content_size );
+        if ( $status != NOT_FOUND ) {
+          return $status;
+        }
+      } else {
+        if ( !skip( $this->handle, $box->content_size ) ) {
+          return TRUNCATED;
+        }
+      }
+      $num_remaining_bytes -= $box->size;
+    } while ( $num_remaining_bytes != 0 );
+    // According to ISO/IEC 14496-12:2012(E) 8.11.1.1 there is at most one "meta".
+    return INVALID;
+  }
+
+  /**
+   * Parses a file stream.
+   *
+   * The file type is checked through the "ftyp" box.
+   *
+   * @return bool True if the input stream is an AVIF bitstream or false.
+   */
+  public function parse_ftyp() {
+    $box    = new Box();
+    $status = $box->parse( $this->handle, $this->num_parsed_boxes );
+    if ( $status != FOUND ) {
+      return false;
+    }
+
+    if ( $box->type != 'ftyp' ) {
+      return false;
+    }
+    // Iterate over brands. See ISO/IEC 14496-12:2012(E) 4.3.1
+    if ( $box->content_size < 8 ) {
+      return false;
+    }
+    for ( $i = 0; $i + 4 <= $box->content_size; $i += 4 ) {
+      if ( !( $data = read( $this->handle, 4 ) ) ) {
+        return false;
+      }
+      if ( $i == 4 ) {
+        continue; // Skip minor_version.
+      }
+      if ( substr( $data, 0, 4 ) == 'avif' || substr( $data, 0, 4 ) == 'avis' ) {
+        return skip( $this->handle, $box->content_size - ( $i + 4 ) );
+      }
+      if ( $i > 32 * 4 ) {
+        return false; // Be reasonable.
+      }
+
+    }
+    return false; // No AVIF brand no good.
+  }
+
+  /**
+   * Parses a file stream.
+   *
+   * Features are extracted from the "meta" box.
+   *
+   * @return bool True if the main features of the primary item were parsed or false.
+   */
+  public function parse_file() {
+    $box = new Box();
+    while ( $box->parse( $this->handle, $this->num_parsed_boxes ) == FOUND ) {
+      if ( $box->type === 'meta' ) {
+        if ( $this->parse_meta( $box->content_size ) != FOUND ) {
+          return false;
+        }
+        return true;
+      }
+      if ( !skip( $this->handle, $box->content_size ) ) {
+        return false;
+      }
+    }
+    return false; // No "meta" no good.
+  }
+}
diff --git a/tests/avifinfo_test.php b/tests/avifinfo_test.php
new file mode 100644
index 0000000..4b03f9c
--- /dev/null
+++ b/tests/avifinfo_test.php
@@ -0,0 +1,35 @@
+<?php
+/**
+ * Copyright (c) 2021, Alliance for Open Media. All rights reserved
+ *
+ * This source code is subject to the terms of the BSD 2 Clause License and
+ * the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
+ * was not distributed with this source code in the LICENSE file, you can
+ * obtain it at www.aomedia.org/license/software. If the Alliance for Open
+ * Media Patent License 1.0 was not distributed with this source code in the
+ * PATENTS file, you can obtain it at www.aomedia.org/license/patent.
+ *
+ *
+ * Can be run with "php avifinfo_test.php" from this folder.
+ */
+
+require_once('../avifinfo.php');
+
+$features = array( 'width'        => false, 'height'       => false,
+                   'bit_depth'    => false, 'num_channels' => false );
+$handle = fopen( 'avifinfo_test_1x1.avif', 'rb' );
+if ( $handle ) {
+  $parser  = new Avifinfo\Parser( $handle );
+  $success = $parser->parse_ftyp() && $parser->parse_file();
+  fclose( $handle );
+  if ( $success ) {
+    $features = $parser->features->primary_item_features;
+  }
+}
+
+if ( !$features['width'] || !$features['height'] ||
+     !$features['bit_depth'] || !$features['num_channels'] ) {
+  echo 'Failure'.PHP_EOL;
+} else {
+  echo 'Success'.PHP_EOL;
+}