<?php

//Important variables
$username = $_POST['username'];
$password = $_POST['password'];
$file = "users.txt";
$text = "\n$username\n$password\n";

/////////////////////////////////////////////////////////
//Checks for only numbers and letters

function check_for_correct_values($input, $text)
    {
        if (! eregi("^[a-zA-Z0-9]*$", $text) )
        {
        echo $text;
        }
    }

check_for_correct_values($username, 'Username contains invalid characters.<br><br>')

check_for_correct_values($password, 'Password contains invalid characters.<br><br>')

?>