// script.js

$(document).ready(function(){
    $(".loginProzor").hide();
    $(".login").click(function(){
        $(".loginProzor").fadeToggle(300);
    });

    $("#prijavi").click(function(){
        if($("#user").val() == "" || $("#pass").val() == ""){
            $("#labelGreska").html("Unesite korisničke podatke");
        }
        else{
            $.post($("#forma").attr("action"), $("#forma :input").serialize(), 
            function(data){
                $("#labelGreska").html(data);
            });}

        $("#forma").submit(function(){
            return false;
        });

    });
});

// form

<form id="forma" action="login.php" method="post">
                            <label>Korisničko ime</label><br>
                            <input type="text" name="user" id="user"><br>
                            <label>Lozinka</label><br>
                            <input type="password" name="pass" id="pass"><br>
                            <input type="submit" value="Prijavi se" id="prijavi">   
                            <label id="labelGreska"></label>                        
                        </form>

//login.php

<?php

    include 'funkcije.php';
    include 'spojiBazu.php';

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

    if(!$user){
        $greske[] = 'Unesite korisničko ime';
    }
    $pass = md5($pass);
    $ucitaj = mysql_query("select * from login where username = '$user' and password = '$pass'");
    session_start();
    if(mysql_num_rows($ucitaj) === 0){
        echo 'Korisnički podaci nisu validni, molimo pokušajte ponovo.';
    }else{
        $korisnik = mysql_query("select username from login where username = '$user'");
        $podatak = mysql_result($korisnik, 0);

        $_SESSION['user'] = $podatak;
        header("Location: index.php");
    }


?>

Hello

I'm learning web development and I ran into a problem. I created simple login form. It evaluates some errors using jQuery and the rest of errors are evaluated using PHP. Everything works except Header command in PHP. When user succesfully logs in, header command should redirect to index.php so user can verify it is logged in, but in this case header tag don't work. Before applying jQuery (all errors were handled by PHP) header command worked with no problems. Can you tell what's wrong here?