判断Gif图片是否是动态GIF (Animated Gif)

PHP的GD库，没有办法直接判断一张gif图片是否是动态的，其gif相关的函数操作的都是动态gif图片的第一帧。

imagecreatefromgif函数的文档中提及了动态gif的判断，即数"frame header"的个数。

//an animated gif contains multiple "frames", with each frame having a
//header made up of:
// * a static 4-byte sequence (\x00\x21\xF9\x04)
// * 4 variable bytes
// * a static 2-byte sequence (\x00\x2C) (some variants may use \x00\x21 ?)

下面的方法来自PHP文档，分区块读取数据，正则表达式判断。

function is_animated_gif($filename){
  if(!($fh = @fopen($filename, 'rb')))  return false;
  $count = 0;
  while(!feof($fh) && $count < 2) {
    $chunk = fread($fh, 1024 * 100); //read 100kb at a time
    $count += preg_match_all('#\x00\x21\xF9\x04.{4}\x00(\x2C|\x21)#s', $chunk, $matches);
  }
  fclose($fh);
  return $count > 1;
}

但是缺点是不能解决frame header跨区块的情况，正则表达式的速度也略慢一些。

下面是自己的修正实现

function is_animated_gif($filename){
  if(!($fh = @fopen($filename, 'rb')))  return false;
  $count = 0;
  while(!feof($fh) && $count < 2) {
    $chunk = fread($fh, 1024 * 100 + 9); //read 100kb at a time
    $size  = strlen($chunk);
    $offset = 0;
    while($count<2){
      $pos = strpos($chunk, "\x00\x21\xF9\x04", $offset);
      if(false===$pos){
	if(!feof($fh)) fseek($fh, -3, SEEK_CUR); // next chunk
        break;
      }else{
	if($pos<($size-9)){
	  if(   substr_compare($chunk, "\x00\x2c", $pos+8,2)==0
	     || substr_compare($chunk, "\x00\x21", $pos+8,2)==0
	    ) {
	      $count++;
              $offset = $pos+10;
	    }else{
              $offset = $pos+4;
            }
	}else{
	  fseek($fh, strlen($chunk)-$pos-1, SEEK_CUR); // next chunk
          break;
        }
      }
    }
  }
  fclose($fh);
  return $count > 1?1:0;
}