// form

<form action="tld-check.php" method="post" id="check">
  <input type="text" name="domain" size="18"> 
  <select name="tld" size="1">
     <option value="com" selected>com</option>
     <option value="net">net</option>
     <option value="org">org</option>
     <option value="org">biz</option>
  </select>
</form>

// $domain var

	$domain = $_REQUEST['domain'] . '.' . $_REQUEST['tld'];