$url = 'http://google.com';
list($status) = get_headers($url);
if (strpos($status, '404') !== FALSE) {
   // URL is 404ing
}

//checking if the site exists by fopen, instead of file_get_contents to speed it up

$url = "URL"; //your url goes in this place instead of nabtron.com

if (@fopen($url,"r")) {
echo "<b>".$url."</b> is accessible<br />";
}