you can lock, unlock, lock and unlock again

It's also useful to know that you can use session_start() and session_write_close() multiple times within a script. So you can, for example, get values to use at the start of the script and then change them later without locking the session in between by doing something like:

session_start();
$loggedUser=$_SESSION['user'];
$orderID=$_SESSION['orderID'];
$orderStatus=$_SESSION['orderStatus'];
session_write_close();

sleep(5); // do a load of stuff

session_start();
$_SESSION['orderStatus']='calculated';
session_write_close();

Perfect point i just added a new action to test this as an answer for another question about centralizing the call to session vars.

As i was saying the only thing need to be considered is the synchronization.

You can review the example over here https://github.com/eslam-mahmoud/PHP-session-locking-problem-with-Ajax