how to check if a url is an image url with php?

You can send a HEAD request to the server and then check the Content-type. This way you at least know what the server "thinks" what the type is.

You can check if a url is an image by using the getimagesize function like below.

function validImage($file) {
   $size = getimagesize($file);
   return (strtolower(substr($size['mime'], 0, 5)) == 'image' ? true : false);  
}

$image = validImage('http://www.example.com/image.jpg');
echo 'this image ' . ($image ? ' is' : ' is not') . ' an image file.';

i think that the idea is to get a content of the header url via curl

and check the headers

After calling curl_exec() to get a web page, call curl_getinfo() to get the content type string from the HTTP header

look how to do it in this link :

http://nadeausoftware.com/articles/2007/06/php_tip_how_get_web_page_content_type#IfyouareusingCURL

Can use this:

$is = @getimagesize ($link);
if ( !$is ) $link='';
elseif ( !in_array($is[2], array(1,2,3))   ) $link='';
elseif ( ($is['bits']>=8) ) $srcs[] = $link;

Here is a way that requires curl, but is faster than getimagesize, as it does not download the whole image. Disclaimer: it checks the headers, and they are not always correct.

function is_url_image($url){
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, $url );
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt($ch, CURLOPT_HEADER, 1);
        curl_setopt($ch, CURLOPT_NOBODY, 1);
        $output = curl_exec($ch);
        curl_close($ch);

        $headers = array();
        foreach(explode("\n",$output) as $line){
            $parts = explode(':' ,$line);
            if(count($parts) == 2){
                $headers[trim($parts[0])] = trim($parts[1]);
            }

        }

        return isset($headers["Content-Type"]) && strpos($headers['Content-Type'], 'image/') === 0;
    }

$ext = strtolower(end(explode('.', $filename)));
switch($ext)
{
case 'jpg':
///Blah
break;
}

Hard version (just trying)

//Turn off E_NOTICE reporting first
if(getimagesize($url) !== false)
{
//Image
}