Обработка формы вызывает сбой apache?

Я использую следующий код для регистрации пользователей на моем сайте. Проблема в том, что когда пользователь регистрирует apache, он не отвечает и вылетает.

Есть ли сбой в моем коде или что-то я делаю не так ????

<?php

include ('../includes/db_connect.php');

$firstname = $_POST['firstname'];
$email = $_POST['email'];    
$username = $_POST['username'];
$password = md5($_POST['password']);

// lets check to see if the username already exists

$checkuser = mysql_query("SELECT username FROM users WHERE username='$username'");

$username_exist = mysql_num_rows($checkuser);

if($username_exist > 0){
    echo "I'm sorry but the username you specified has already been taken.  Please pick another one.";
    unset($username);
    //include 'register.html';
    exit();
}

// lf no errors present with the username
// use a query to insert the data into the database.

$query = "INSERT INTO users (firstname, email, username, password)
VALUES('$firstname', '$email', '$username', '$password')";
mysql_query($query) or die(mysql_error());
mysql_close();

echo "You have successfully Registered";

// mail user their information

//$yoursite = ‘www.blahblah.com’;
//$webmaster = ‘yourname’;
//$youremail = ‘youremail’;
//    
//$subject = "You have successfully registered at $yoursite...";
//$message = "Dear $firstname, you are now registered at our web site.  
//    To login, simply go to our web page and enter in the following details in the login form:
//    Username: $username
//    Password: $password
//    
//    Please print this information out and store it for future reference.
//    
//    Thanks,
//    $webmaster";
//    
//mail($email, $subject, $message, "From: $yoursite <$youremail>\nX-Mailer:PHP/" . phpversion());
//    
//echo "Your information has been mailed to your email address.";

?>