Simple register form

function register(){
        
        
        switch($_GET['op']){
    
        case 'submit':
        $clean = array();
        $mysql = array();
    
        $now = time();
        $max = $now - 15;    
        $salt = $sitekey;
        $email = $_POST['email'];
        
        if (ctype_alnum($_POST['username'])){
          $clean['username'] = $_POST['username'];
        
        }
            
        $clean['password'] = md5($salt . md5($_POST['password'] . $salt));
        $mysql['username'] = mysql_real_escape_string($clean['username']);
        $uname = $mysql['username'];
        $upass = $clean['password']; 
        $ip_address = $_SERVER['HTTP_REFERER'];
        $sql = "INSERT into users(username, password, email, ip_address)
                VALUES ('$uname','$upass','$email', '$ip_address')";
        $result = mysql_query($sql) or die('error inserting data');
        break;
        
        default:
        echo "<form action=\"login.php?action=register&op=submit\" method=\"post\" enctype=\"application/x-www-form-urlencoded\">
                  <label>Username<br>
                  <input type=\"text\" name=\"username\">
                  </label>
                  <br>
                  <label>Password<br>
                  <input type=\"password\" name=\"password\">
                  </label>
                  <br>
                  <label>Email<br>
                  <input type=\"text\" name=\"email\">
                  </label>
                  <br>
                  <label>
                  <input type=\"submit\" name=\"Submit\" value=\"Register\">
                  </label>
                </form>";
        break;
    }

}