จะแก้โค๊ดฟังก์ชัน str_replace ของเดิมให้สั้นๆกว่าเดิมได้มั้ยคะ

$domain = explode(".", $_POST['domain_check']);
$domain_check = "$domain[1].$domain[2]";

function domain_check($domain){
        $ex_domain=explode(".", $domain);
   return $ex_domain[1].'.'.$ex_domain[2];
}


$domain_check = domain_check($_POST['domain_check']);

<?php
function extract_domain($value)
{
	return preg_match('/^(?>(?>http[s]?\:)?\/\/)?(?>www\.)?([^\s\/$]+)/i', trim($value), $matches)
		? $matches[1]
		: '';
}
echo extract_domain('w3.org'); // w3.org
echo extract_domain('http://127.0.0.1'); // 127.0.0.1
echo extract_domain('http://localhost'); // localhost
echo extract_domain('http://msn.com'); // msn.com
echo extract_domain('https://www.facebook.com'); // facebook.com
echo extract_domain('https://www.thaicreate.com/php/forum.html'); // thaicreate.com