PHP crack anti-hotlink image function_PHP tutorial

$url=$_GET['url'];
$ext=pathinfo($url,PATHINFO_EXTENSION);
if($ext!='jpg' && $ext!='gif'){ // Only jpg and gif are supported
readfile('/ upload/20081209130557536.gif');
exit;
}
$file=md5($url).'.'.$ext;
if(file_exists($file)){
readfile($file);
exit;
}else{
$data=file_get_contents($url);
if(!$data){ // Failed to read
readfile( '/upload/20081209130557536.gif');
exit;
}
$handle=fopen($file,'wb');
fwrite($handle,$data);
fclose($handle);
echo $data;
}
?>