Save $_SESSION on JQUERY $.POST - php

Question

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>