Problem so Session

<?php
include "overenie.php";
?>

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

$nick = $_SESSION["nick"];
$heslo = $_SESSION["heslo"];

if(!$nick || !$heslo)
{
echo "
    <script>
    window.location.replace(\"index.php?url=admin_error3\");
    </script>";
exit();
}
Else
{
$sql = mysql_query("select * from texty where text1 = '$nick' and text2 = '$heslo'");
$row = mysql_num_rows($sql);
If($row !=1) {

echo "
    <script>
    window.location.replace(\"index.php?url=admin_error2\");
    </script>";


exit(); }
}
?>

<?php
$chyba = $_GLOBALS['error'];
If($chyba==1) { echo "<p><b>Meno a heslo nesúhlasí</b></p>";}
Elseif($chyba==2) { echo "<p><b>Nevyplnil si meno alebo heslo</b></p>";}
Elseif($chyba==3) { echo "<p><b>Ak ste bol dlhšiu dobu neaktivný, systém vás odhlásil</b></p>";}
?>
<p>
<form method="post" action="login.php">
Meno <input name="nick"> <br>
Heslo <input name="heslo" type="password">
<input type="submit" value="Prihlásit">
</form>
</p>

<?php
session_start();
include ("config.php");


$nick = $_POST["nick"];
$heslo= $_POST["heslo"];
If(!$nick || ! $heslo) {
echo "
    <script>
    window.location.replace(\"index.php?url=admin_error2\");
    </script>";
exit();
}

$sql = mysql_query("select * from texty where text1 = '$nick' and text2 = '$heslo'");
$row = mysql_num_rows($sql);
If($row==1)
{
$_SESSION["nick"]= $nick;
$_SESSION["heslo"] = $heslo;
echo "
    <script>
    window.location.replace(\"index.php?url=admin\");
    </script>";
exit();
}
Else
{
echo "
    <script>
    window.location.replace(\"index.php?url=admin_error1\");
    </script>";
exit();
}
?>

If(!$nick || ! $heslo)

md5(sha1($premenna));

http://binnyva.blogspot.com/2005/11/md5-is-dead-use-sha1.html

http://webreflection.blogspot.com/2007/01/javascript-unobtrusive-security-form.html