<?php
                $username = $_POST['textid'];
                $password = $_Post['textid2'];
                $usernames = {
                    'username1',
                    'username2',
                    'username3'
                }
                $passwords = {
                    'pw1',
                    'pw2',
                    'pw3'
                }
                if (isset($_POST['Submit1'])) {
                    if($username == $usernames && $password == $passwords) {}
                }
            ?>