在JQUERY $ .POST上保存$ _SESSION

I have an HTML page in which I use Jquery .post() to post the forms into one big php file. I thought that this way I'll be able to save the username in a $_SESSION. I have a login form that stores the username into a session, but then, when I try to display the php file (using .load()) in a different html page (linked to by the login one) I have no session variable set. When I did a little debugging I found out the session is lost only if it's created in the if(isset($_POST['username'})) part of the page.

<?php
    session_start();
    include_once("connect.php");
    $username = $_SESSION['username'];

    //check if username is in session
    if(isset($_SESSION['username'])) {
        $username = $_SESSION['username'];
        echo "<script>$(document).ready(function(){location.replace('index.html#home')});</script>";    
    }else{
?>
<div id="part_login">
<?php
    //if not, check login
        if(isset($_POST['username']) and isset($_POST['password'])){
            $username = mysqli_real_escape_string($conn, $_POST['username']);
            $password = md5($_POST['password']);

            $result = mysqli_query($conn, "SELECT * FROM users WHERE username='$username' AND password='$password' AND active='1' LIMIT 1") or die(mysqli_error($conn));
            $count = mysqli_num_rows($result);
            if ($count == 1){
                $_SESSION['username'] = $username;

                $result = mysqli_query($conn, " SELECT * FROM `users` WHERE username = '$username'");
                while($row = mysqli_fetch_array($result)) {
                    $oldlogin = $row['lastlogin'];
                }

                echo "<script>$(document).ready(function(){location.replace('index.html#home')});</script>";
            }else{
                echo "<div class='ui-btn'>Invalid Login Credentials</div>";
            }
        }
    }

        if (isset($_SESSION['username'])){
            $username = $_SESSION['username'];

            $result = mysqli_query($conn, " SELECT * FROM `users` WHERE username = '$username'");
                while($row = mysqli_fetch_array($result)) {
                    $oldlogin = $row['lastlogin'];
                }

            $query = mysqli_query($conn, "UPDATE `users` SET lastlogin=now() WHERE username='$username' LIMIT 1");
        }else{  
        }
?>
</div>
<div id="part_user">
<?
    echo $username . " " . $oldlogin;
?>
</div>