<form action="script.php" method="post">
<input type="text" name="username" value="$username" />
<input type="text" name="password" value="$password" />
<input type="submit" name="submit" />
</form>

<?php

require_once("mysql.php");

if (!empty($_POST['username']) && !empty($_POST['password']) {

$username = $_POST['username'];
$password = $_POST['password'];

mysql_query("INSERT into USERS (username,password) VALUES ('$username', '$password')");

}

else "Go back and fill out the form.";

?>