Checking if user exists

[parat26]

Ok guys, these weeks I have started developing a browser game since my PHP skills have developed. I am stuck at checking if username already exists.

The registration works perfect however I am able to register multiple accounts with the same name and I can't figure out whats the problem in my code:

Code:

<?php
include("../config/database.config.php");

Function Register()
{
    $conn = mysql_connect(HOST,USER,PASS);
    mysql_select_db(DATABASE,$conn);

    $username    = mysql_real_escape_string(trim($_POST['txtusername']));
    $password    = mysql_real_escape_string($_POST['txtpassword']);
    $email        = mysql_real_escape_string(trim($_POST['txtemail']));

    $username = stripslashes($username);
    $password = stripslashes($password);
    $email = stripslashes($email);

    $checkuser = mysql_query("SELECT * FROM users WHERE username = '". $username ."' OR email = '". $email ."'");

    // $password    = md5($password);
    $salt = sha1(md5($password));
    $password = md5($password.$salt);

    if (mysql_num_rows($checkuser) > 0 && empty($_POST['txtusername']) == true && empty($_POST['txtpassword']) == true && empty($_POST['txtemail']) === true)
    {

        header('location:../index.php?p=error');

    } else {
        $query ="insert into users(username,password,email)values('$username','$password','$email')";
        $res = mysql_query($query);
        header('location:../index.php?p=success');    
    }
}

Register();

Any suggestions?