无法使用php将数据插入数据库

Question

I'm trying to validate my code, but then I can't insert the data in the database! Can someone tell me why? I tried it just for one variable so I could see if it will work but no success

<?php

 require('connection/connection.php');

 if(isset($_POST['submit'])) {
    $firstname = $_POST['firstname'];
    $lastname =$_POST['lastname'];
    $email = $_POST['email'];
    $tel = $_POST['tel'];
    $password = $_POST['pass'];
    $flat =$_POST['flat'];
    $num =$_POST['num'];
    $street =$_POST['street'];
    $district =$_POST['district'];
    $selected_val = $_POST['city'];

    mysql_query('SET NAMES utf8');

    //form validation.
    if(empty($_POST['firstname'])){

        echo "qq";
    } else{
        $query = "INSERT INTO `parenuser` 
                    (flat, num, street, district, 
                     city, firstname, lastname, tel, 
                     pass, email) 
                  VALUES 
                    ('$flat', '$num', '$street', '$district', 
                     '$selected_val', '$firstname', '$lastname', '$tel', 
                     '$password', '$email')";


        $result = mysql_query($query);

        if($result){

            header("Location: success.html");

        }

    }

}
?>