PHP で GIF を読む

is_animated_gif.php

<?php
class GIFReader
{

    const TRAILER = 0x3b;

    const IMAGE_INTRODUCER = 0x2c;
    const EXTENSION_INTRODUCER = 0x21;

    const GRAPHIC_CONTROL_LABEL = 0xf9;
    const COMMENT_CONTROL_LABEL = 0xfe;
    const PLAIN_TEXT_LABEL= 0x01;
    const APPLICATION_EXTNSION_LABEL = 0xff;

    const BLOCK_TERMINATOR = 0x00;

    public static function isAnimatedGif($filename)
    {
        $f = fopen($filename, 'r');

        $signature = fread($f, 3);
        $version = fread($f, 3);

        if (!in_array($signature . $version, array('GIF89a', 'GIF87a'), true)) {
            return false;
        }

        $screen_width = ord(fread($f, 2));
        $screen_height = ord(fread($f, 2));

        $gctf_info = ord(fread($f, 1));
        $background_color_index = ord(fread($f, 1));
        $pixel_aspect_ratio = ord(fread($f, 1));

        if ($gctf_info >> 7) {
            fread($f, 3 * (2 << ($gctf_info & 0b111)));
        }

        $frame_num = 0;
        while (1) {
            $introducer = ord(fread($f, 1));
            if ($introducer == self::TRAILER) {
                break;
            }

            if ($introducer == self::IMAGE_INTRODUCER) {
                if (++$frame_num == 2) {
                    return true;
                }

                $image_left_position = fread($f, 2);
                $image_top_position = fread($f, 2);
                $image_width = fread($f, 2);
                $image_height = fread($f, 2);

                $lctf_info = ord(fread($f, 1));
                if ($lctf_info >> 7) {
                    fread($f, 3 * (2 << ($lctf_info & 0b1111)));
                }
                fread($f, 1);
                while (($size = ord(fread($f, 1))) != self::BLOCK_TERMINATOR) {
                    fread($f, $size);
                }
            }

            if ($introducer == self::EXTENSION_INTRODUCER) {

                $label = ord(fread($f, 1));

                switch ($label) {
                    case self::APPLICATION_EXTNSION_LABEL:
                        while (($size = ord(fread($f, 1))) != self::BLOCK_TERMINATOR) {
                            fread($f, $size);
                        }
                        break;
                    case self::GRAPHIC_CONTROL_LABEL;
                        while (($size = ord(fread($f, 1))) != self::BLOCK_TERMINATOR) {
                            fread($f, $size);
                        }
                        break;
                    case self::COMMENT_CONTROL_LABEL:
                    case self::PLAIN_TEXT_LABEL:
                        while (($size = ord(fread($f, 1))) != self::BLOCK_TERMINATOR) {
                            fread($f, $size);
                        }
                        break;
                    default:
                        throw new Exception("Broken GIF");
                        break;
                }

            }

        }

        fclose($f);

        return false;
    }
}

$file = $argv[1];
if (GIFReader::isAnimatedGif($file)) {
    echo "animated gif" . PHP_EOL;
} else {
    echo "not animated gif" . PHP_EOL;
}