sessions not working - php

Question

So I had developed a basic site, using $_SESSION superglobal variable for the logging in.
so the code basically after checking the login details are valid i store the users details into the session like so:
note I am starting the session before storing these values.
$_SESSION['myusername'] = $myusername;
$_SESSION['myuserid'] = $userid;
$_SESSION['logged_in'] = true;
$_SESSION['mystatus'] = $res['user_status'];
it all worked fine, throughout the time i made the site and tested etc.
now all of a sudden, the sessions are not working, so obviously the users cannot get access after logging in because the site is checking data which isnt in the session.
on the page I store the data like above, straight after i can use this:
echo "username".$_SESSION['myusername'];
echo "status".$_SESSION['mystatus'];
and its there. But when the user is directed to another page and i try:
<?php
session_start();
include ('functions.php');
echo "username".$_SESSION['myusername'];
echo "status".$_SESSION['mystatus']; ....
the values aren't in the session. I have checked that the session id is the same, which it is.
This has always worked, so I am really puzzled.
somebody please help.
EDIT
request header & response header from firebug (page where session appears to be empty)
Response Headersview source
Date Sat, 11 Jun 2011 15:18:48 GMT
Server Apache/2.2.3 (Red Hat)
X-Powered-By PHP/5.1.6
Expires Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma no-cache
Content-Length 3772
Connection close
Content-Type text/html; charset=UTF-8
Request Headersview source
Host students.ee.port.ac.uk
User-Agent Mozilla/5.0 (Windows; U; Windows NT 6.1; en-GB; rv:1.9.2.10) Gecko/20100914 Firefox/3.6.10
Accept text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language en-gb,en;q=0.5
Accept-Encoding gzip,deflate
Accept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive 115
Connection keep-alive
Cookie PHPSESSID=1jqqa2oeivq76h2vhtk4uflkv1
Authorization Basic ZWNlNzAxNDE6cGllczRtZTIy

Answer 1

So it seems you have a problem with keeping your session on a second request.
Session tracking is done via cookies, you should check (with Live HTTP Headers or firebug) the real cookie content sent by the server. In this cookie check the path setting and the server name given, check as well time validity settings, if something is wrong there the browser won't send back the cookie and you'll get a new session on each request.
The web developper Toolbar contains some nice cookie tools as well, where you can display a page containing all cookies details for a given page. If the cookie receveid is not there then the browser assume this cookie is not related to this page. Most of the time a php setting is enforcing the cookie.domain setting to something other than the used DNS.

Answer 2

Given the fact that you haven't changed a thing in the last few weeks and that it used to work, you should check that your server didn't run out of disk space. If it did, it may create a reference to a session but might not be able to serialize the data to disk once the page has been rendered.
This could explain why outputting the $_SESSION[...] works on the same page and why the cookie is set in the response.

Answer 3

Check whether the session id on the second request is the same as the one on the first request.