Ceriwis - [ASK] PHP gan... masalah username sama di registrasi

mau nanya ni gan... penting banget....

PHP Code:

$username = mysql_real_escape_string($_POST['username']);

$cariuser = "select * from tbl_user where 'username' = '".$_POST['username']."' ";

$user = mysql_query($cariuser) or die("Query failed with error: ".mysql_error());

$tampiluser = mysql_fetch_array($user,MYSQL_NUM);

$jumlah = mysql_num_rows($user);

$row = mysql_fetch_assoc($user);

$a = 0;

error_reporting(0);

if (empty($_SESSION['username']) AND empty($_SESSION['passuser'])){

//Jika Tidak login, maka lempar ke index utk login dulu =========

echo "";

echo "";

}

//mencari kesamaan username pada database

if($tampiluser = 1 ){

echo "";

echo "";

}

else{

$nama = $_POST['nama'];

$username = $_POST['username'];

$password = $_POST['password'];

$passenkrip = md5($password);

nah ini sourcenya.... critanya itu kan buat nyari username yg sama.. tp pas masukin username sama apa g sama langsung kena alert semua... gmn ya solusinya..????

</div>