PHP: check domain availability

function check_domain($domain) {
if(preg_match('/[;\&\|\>\<]/', $domain)) exit;

exec("whois " . escapeshellarg($domain), $output);
$result = implode("\n", $output);

return (strpos($result, 'No match') !== false);
}
if( check_domain("domain.com")){
//domain.com still available
}