Hvorfor virker dette ikke ?

Kode 

<?
/* Check User Script */
session_start();  // Start Session
include 'mysql.php';
// Conver to simple variables
$username = $_POST['username'];
$password = $_POST['password'];
if((!$username) || (!$password)){
    echo "Please enter ALL of the information! <br />";
    exit();
}
// Convert password to md5 hash
$password = md5($password);
// check if the user info validates the db
$sql = mysql_query("SELECT * FROM admin WHERE username='$username' AND password='$password'");
$login_check = mysql_num_rows($sql);
if($login_check > 0){
    while($row = mysql_fetch_array($sql)){
    foreach( $row AS $key => $val ){
        $$key = stripslashes( $val );
    }
        // Register some session variables!
        session_register('first_name');
        $_SESSION['first_name'] = $first_name;
        session_register('last_name');
        $_SESSION['last_name'] = $last_name;
        session_register('email_address');
        $_SESSION['email_address'] = $email_address;
        session_register('special_user');
        $_SESSION['user_level'] = $user_level;
        header("Location: demo_strationer.php");
    }
} else {
    echo "You could not be logged in! Either the username and password do not match or you have not validated your membership!<br />
    Please try again!<br />";
}
?>
		
		<h1 align="center">Login Tak! </h1>
		<p> </p>
		<table width="200" border="0" align="center" cellpadding="0" cellspacing="0">
          <tr>
            <td><form action="<?php $_SERVER['PHP_SELF'] ?>" method="post">
		<strong>Brugernavn:</strong> <input type="text" name="username"/><br />
		<strong>Password:</strong> <input type="password" name="password" class="input" /><br />
		<br />
		<br />
		<div align="center"><input type="submit" name="login" value="Login" />
		</div>
		<br />
            </form></td>
          </tr>
        </table>