Login problem

<?php
ob_start();
session_start();
error_reporting(E_ALL);
if(isset($_POST['nick'])){
$nick=$_POST['nick'];
$pass=$_POST['heslo'];
$passMD5=md5($pass);
$ip=$REMOTE_ADDR;
$ip_full=gethostbyaddr($ip);
$date=date("jnYGi");
if($nick=="" || $pass==""){
echo "<center><div class=\"cont-title\">Priebeh prihlasovania</div></center>";
echo "<b>Vyplňte prosím všetky údaje!</b>";
}else{
require_once "config.php";
$is_banned=mysql_query("SELECT * FROM `refresh_users` WHERE `nick`='$nick'") or die (mysql_error());
$is_banned_row=mysql_fetch_array($is_banned);
if($date=>$is_banned_row['banned_to']){
$delte_ban=mysql_query("UPDATE `refresh_users` SET `banned_by`='', `banned_reason`='', `banned_to`='' WHERE `nick`='$nick'") or die (mysql_error());
}else{
echo "<center><div class=\"cont-title\">Priebeh prihlasovania</div></center>";
echo "<b>Tento účet je zabanovaný!</b><br>Ban udelil administrátor <u>".$is_banned_row['banned_by']."<u>.<br>Dôvod: ".$is_banned_row['banned_reason']."</b>";
echo "O unban môžete požiadať cez <a href=\"index.php?a=unban\">tento formulár</a>.";
}
$user_exists=mysql_query("SELECT * FROM `refresh_users` WHERE `nick`='$nick' AND `heslo`='$passMD5'") or die (mysql_error());
$row=mysql_fetch_array($query);
if(!empty($row['nick'])){
$_SESSION['logged']=1;
$_SESSION['nick']=$row['nick'];
$_SESSION['prava']=$row['prava'];
$_SESSION['id']=$row['id'];
echo "<center><div class=\"cont-title\">Priebeh prihlasovania</div></center>";
echo "<b>Prihlásenie prebehlo úspešne!</b>";
}else{
echo "<center><div class=\"cont-title\">Priebeh prihlasovania</div></center>";
echo "<b>Špatne zadané údaje! Skúste to znovu prosím.</b>";
}
}
}
ob_end_flush();
?>