Accessing a session variables in a php class object fails after php5.3 > php5.5.9 upgrade

Object & session properties are not returned from the class function into the initiating script, even though the classes and objects initiate. I can see their properties with var_dump() from within the class function but they are not returned outside the class function.

I have a globally included file global.inc.php which gets called on every page which does a bunch of things. On user form Login, the include calls a custom class ('UserTools.class.php') using:

$userTools = new UserTools();

This class does 3 things:

1. Database Check on User (some password verification/hashing comes later - omitted here):

public function login($username, $password) {
$result = mysql_query("SELECT * FROM users WHERE username = '$username'");
$row = mysql_fetch_assoc($result);

2. Set a bunch of $_SESSION variables (session_start() already called from global.inc.php)

$_SESSION["login_time"] = time();
$_SESSION["logged_in"] = true;
$_SESSION["passwordHash"] = $passwordHash;

3. Initialize the User class and serialize the User object to the session variable $_SESSION['user']:

$_SESSION["user"] = serialize(new User($row));

If I var_dump($_SESSION['user']) from "within" the class, I can see the serialized string representation of the User object:

var_dump($_SESSION['user']):

array (size=4)
'SESS_PARENT' => boolean true
'SESS_CHILD' => boolean true
'timezone' => int 300
'user' => string 'O:4:"User":13: {s:2:"id";s:1:"1";s:8:"username";s:5:"peter";s:14:"hashedPassword";s:60:"$2y$11.....(length=6160)

THE PROBLEM
But when the class function returns to the calling global.in.php script, the complete $_SESSION variable is:

array (size=0) . empty

Q1. What am I doing wrong here?

Q2. What has changed since php 5.3 to have this effect?

Session_start()

The culprit turned out to be the $_SESSION superglobal and a complex set of includes.

session_start(); was not at the start of the include file, only some way down after some ini_set commands setting up various parameters for the $_SESSION superglobal.

Header Redirects

Header redirects were not saving session data. This is because, any changes to $_SESSION varibles are made when a script ends. A header redirect with an exit() statement is essentially interrupting a script’s execution - so the session needs to be written back to disk, database or Redis 'before' the redirect and exit() commands are called.