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

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;
    }