php怎么判断gif图片是否为动画？

{
$fp=fopen($filename, 'rb'); $filecontent=fread($fp, filesize($filename));
fclose($fp);
return strpos($filecontent,chr(0x21).chr(0xff).chr(0x0b).'NETSCAPE2.0')===FALSE?0:1;
} // bbs.it-home.org
echo IsAnimatedGif("zzsky.gif");
?>