override include php if logged in - php

Question

I'm working on a login system. I want when I login at index.php, the include from that page change to another. So in my case the fully header change and the login button disappear.
So I want to change <?php include('header.php'); ?>
to
<?php include('header2.php'); ?>
if logged in
The login:
<div id="loginContainer">
<span>Login</span><em></em>
<div style="clear:both"></div>
<div id="loginBox">
<form id="loginForm" action="login.php" autocomplete="on" method="post">
<fieldset id="body">
<fieldset>
<label for="email">Email Address</label>
<input id="username" name="username" required="required" type="text" placeholder="myusername"/>
</fieldset>
<fieldset>
<label for="password">Password</label>
<input id="password" name="password" required="required" type="password" placeholder="eg. X8df!90EO" />
</fieldset>
<input type="submit" name="login" value="Login" />
<label for="checkbox"><input type="checkbox" id="checkbox" />Remember me</label>
</fieldset>
<span>Forgot your password?</span>
</form>
</div>
</div></
Part of login.php
if($res = $mysqli->query($q))
{
if($res->num_rows > 0)
{
$_SESSION['userName'] = $userName;
//header("Location:welcome.php");
include('header4.php');
exit;
}
else
{
echo'<script>alert("INVALID USERNAME OR PASSWORD");</script>';
header("Location:index.php");
exit;
}
}

Answer 1

Simply check if a session is set, and then include the right file. Something like this.
if (isset($_SESSION['userName']) && !empty($_SESSION['userName'])) {
// If the user is logged in, include this file
include "header2.php";
} else {
// Else, we're not logged in, include this file
include "header.php";
}