Nefunguje login

session_start();
include "mysql_connect.php";

if($_GET['action'] == "logout"){

	$_SESSION = array(); 

	session_destroy();

	header("Location: index.php?al=4"); 

} 
if($_GET['action'] == login){
	$_POST['nick']=htmlspecialchars($_POST['nick']);
	$_POST['pass']=htmlspecialchars($_POST['pass']);

	$_POST['pass']=sha1($_POST['pass']);

	login($_POST['nick'],$_POST['pass']); 

} 
function login($nick,$pass){ 
    $query = mysql_query( "SELECT `id`,`nick`,`pass`,'jmeno',`prij`,`e_mail` FROM `uzivatele` WHERE (`nick`='$nick') AND (`pass`='$pass')"); 

    $rows = mysql_fetch_array($query); 

    $nick_ch=mysql_num_rows(mysql_query( "SELECT * FROM `uzivatele` WHERE (`nick`='$nick')"));
	if($nick_ch == 0) { $al="3"; }

	if(($pass == "")||($nick == "")) { $al="2"; }  else {

	if (($rows['pass'] != $pass)&&($nick_ch > 0)) { $al="1"; } }

	if(mysql_num_rows($query) > 0){ 

		 $_SESSION['user']['nick'] = $nick; 

		 $_SESSION['user']['pass'] = $pass;

		 $_SESSION['user']['id_user'] = $rows['id'];

		 $_SESSION['user']['meno'] = $rows['jmeno']; 

		 $_SESSION['user']['prij'] = $rows['prij']; 

		 $_SESSION['user']['e_mail'] = $rows['e_mail'];
		 $al = "0";

	        }

	if($al>0) { 

		header("Location: index.php?al=$al");

	} else {

		header("Location: index.php?page=mu_prehled&m=1");

	}

  }
switch($_GET['al']) {

	case "1": $mess_log = "Snaha o neautorizovaný přístup!"; break;

	case "2": $mess_log = "Musíte vyplnit uživatelské jméno a heslo!"; break;

	case "3": $mess_log = "Toto uživatelské jméno nemá nikdo z uživatelů!"; break;

	case "4": $mess_log = "Jste úspěšně odhlášen/a!"; break;

}