codice:
function check_url($url) {
	$ch = curl_init();

	curl_setopt($ch, CURLOPT_URL, $url);
	curl_setopt($ch, CURLOPT_NOBODY, 1);
	curl_setopt($ch, CURLOPT_HEADER, 1);
	curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

	$headers = explode("\n", curl_exec($ch));
	curl_close($ch);

	$response = rtrim($headers[0]);
	if ( !preg_match('/HTTP.*? (\d+)/', $response, $match) )
		return FALSE;

	if ( $match[1] == 200 )
		return TRUE;

	return FALSE;
}

var_dump(check_url('http://www.html.it/'));
var_dump(check_url('http://www.html.xx/'));