index.php

<!doctype html>
<html>
<body>
    <form action="/" method="POST">
        Domain Name
        <input type="text" name="domain_name">
        <input type="submit" value="nslookup">
    </form>
</body>
</html>

<?php

    function is_domain($domain)
    {
        //domain, sub domain, top level domain
        return preg_match('/^ (?: [a-z0-9] (?:[a-z0-9\-]* [a-z0-9])? \. )*[a-z0-9] (?:[a-z0-9\-]* [a-z0-9])?\.[a-z]{2,6}$/ix', $domain);
    }

    if(isset($_POST["domain_name"]) && is_domain($_POST["domain_name"]))
    {
        system("nslookup " . $_POST["domain_name"]);
    }
?>