$webimage = 'http://pcdn.500px.net/5953805/d0dd841969187f47e8ad9157713949b4b95b3bda/4.jpg?1333782904356';
$pieces = explode("/", $webimage); 
$pathend = end($pieces);
$imageinfo = @getimagesize($webimage);
$imagetype= $imageinfo['mime'];
if($imagetype=='image/jpeg'){
    if(stripos($pathend,'.jpg')==false){
        $newpathend = $pathend.'.jpg'; // if image end is't '.jpg', add '.jpg'
    }else if(stripos($pathend,'.jpeg')==0){
        $newpathend = $pathend.'.jpeg'; // if image end is't '.jpg', add '.jpeg'
    }else{
        $newpathend = $pathend;// if image end is '.jpg' or '.jpeg', do not change
    }
}
if($imagetype=='image/png'){
    if(stripos($pathend,'.png')==false){
        $newpathend = $pathend.'.png'; // if image end is't '.png', add '.png'
    }else{
        $newpathend = $pathend;// if image end is '.png', do not change
    }
}
if($imagetype=='image/gif'){
    if(stripos($pathend,'.gif')==false){
        $newpathend = $pathend.'.gif'; // if image end is't '.gif', add '.gif'
    }else{
        $newpathend = $pathend;// if image end is '.gif', do not change
    }
}

$url = "http://pcdn.500px.net/5953805/d0dd841969187f47e8ad9157713949b4b95b3bda/4.jpg?1333782904356";
$url_arr = parse_url($url);
echo basename($url_arr['path']);

------解决方案--------------------

PHP code
$webimage  = 'http://pcdn.500px.net/5953805/d0dd841969187f47e8ad9157713949b4b95b3bda/4.jpg?1333782904356';
$pieces    = explode("/", $webimage);
$fileName   = end($pieces);
$ar = explode('.', $fileName);
preg_match('/^(jpg|jpeg|png|gif){1}\.*/i', $ar[1], $match);
$newFileName = $ar[0].'.'.$match[1];
echo $newFileName;