Website doesn't display pages as requested until Control+F5 - php

Question

I have a strange situation here.
I had coded a webpage in php. It works fine throughout the day however if I close it down and try to access it again the following day it doesn't quite work as needed.
For example:
I have a main login page. When the user logs in the URL changes to the following page but it still shows the login input.
if i manually enter the URL again and press CTRL+F5 I get the original page. If I then press on the logout page which calls the logout.php the URL does change but it still shows me the index page until I manually entere the URL and press CTRL+F5 to force refresh.
once all the pages have been accessed this way the website they starts working fine again.
I have put headers to not cache the webpages but that is not helping here.
It happens with all browsers.
Any advice or am I sounding a bit stupid here?
header("Cache-Control: no-cache, must-revalidate");
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
session_start();
if(isset ($_SESSION['user']))
{
echo "Logged in as:".$_SESSION['user'];
echo "Home";
echo "Logout";
exit;
}
else
{
echo "<FORM name=\"login\" action=\"index.php\" method=\"POST\" onsubmit=\"return validatelogin(this);\">";
echo "<input type=\"text\" placeholder=\"Username\" name=\"username\">";
echo "<input type=\"password\" placeholder=\"Password\" name=\"password\"> ";
echo "Forgot my password";
echo "<input type=\"submit\" name=\"submit\" value=\"Submit\"> ";
echo "</FORM>";