<?php
$url = 'http://www.vg.no/gfk/front/vglogo.gif';
$valid = array('image/gif', 'image/jpeg', 'image/png');

function is_img($url, $valid)
{
    if (!file_exists($url)) return false;
	
	$mime = getimagesize($url);
	$mime = $mime['mime'];
	
	return (in_array($mime, $valid)) ? true : false;
}

if ( is_img($url, $valid) )
{
    echo 'Ett gyldig bilde';
}
else {
    echo 'Et ugyldig bilde';
}
?>

<?php
function is_img($url)
{
   $valid = array('gif', 'jpeg', 'png');
	
	$ch = curl_init();
	curl_setopt($ch, CURLOPT_URL, $url);
	curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
	curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 0);
	curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 10);
	curl_setopt($ch, CURLOPT_TIMEOUT, 10);
	curl_setopt($ch, CURLOPT_NOBODY, true);
	curl_setopt($ch, CURLOPT_HEADER, true);
	
	$header = curl_exec($ch);
	curl_close($ch);
	
	preg_match('#Content\-Type\: image\/([a-z]{3,4})#si', $header, $result);
	
	return ($result[1] && in_array($result[1], $valid)) ? true : false;
}
?>