Adds a user to our database

AddUser.php

<?php
	$db = mysql_connect('localhost', 'username', 'password') or die ('Could not connect: ' . mysql_error());
	mysql_select_db('databasename') or die ('Could not select database');

	$email = mysql_real_escape_string($_GET['email'], $db);
	$username = mysql_real_escape_string($_GET['username'], $db);
	$password = mysql_real_escape_string($_GET['password'], $db);
	$hash = $_GET['hash'];

	$secretKey = 'secretkey';

	$real_hash = md5($email . $username . $password . $secretKey);
	if ($real_hash == $hash)
	{
		$query = "INSERT INTO users (username, password, creationdate, email) values ('$username', '$password', NOW(), '$email');";
		$result = mysql_query($query) or die('Query failed: ' . mysql_error());
	}
?>