<?
$user_name = $_REQUEST['user_name'];
$pass = $_REQUEST['pass'];
$date_now = date("Y")."/".date("m")."/".date("d"); //date("d/m/Y h:i");
mysql_connect("localhost","Xioottye","TKKLLAA4") or die(mysql_error());
mysql_select_db("Xioottye") or die(mysql_error());
mysql_query("INSERT INTO tabel_test(user_name,pass,date_now) values('$user_name','$pass','$date_now')");
$query = mysql_query("SELECT user_name FROM tabel_test WHERE user_name ='$user_name' ") or die(mysql_error());
//aqui ves, si hay coincidencias
if (mysql_num_rows($query) > 0){ //si es mayor a cero existe tu registro
print "<table border=10>";
print "<tr><td> EXISTE ......! </td></tr>";
print "</table>";
}else{
mysql_query("INSERT INTO tabel_test(user_name,pass,date_now) values('$user_name','$pass','$date_now')");
}
?>
Espero que te sirva.