PHP Sessions' internal mechanics - php

Question

From the manuals, I draw that when a php session var is set, it is written to a text file in the session_save_path folder.
I am just curious to find out if this happens as soon as the interpreter hits the line with the session variable or does it ( writing to the text file ) takes place when the PHP interpreter exits processing the file?
For example, if I were to set and update a session variable in two consecutive lines, (as in the example I gave below), does the PHP interpreter saves the files twice back to back?
In other words, which code snippets has the right commenting?
$_SESSION['my_variable']=1; // writes to the session text file
$_SESSION['my_variable']=2; // writes to the session text file again
die(); //
versus
$_SESSION['my_variable']=1; // updates the session file contents in the memory
$_SESSION['my_variable']=2; // updates the session file contents in the memory
die(); // writes to the session text file

Answer 1

Data is written to file if:
session_write_close() is called
the script execution is finished
So provided that you don't do 1), your second assumption is correct.
Writing to the file every time a variable changes would be VERY expensive, because generally speaking accessing and writing to the disk is slow, hence why PHP won't do that. It should be noted however that caching systems such as memcache or redis will store changes as they happen, so it can be a good idea to rely on them when PHP sessions do not suffice in terms of reliability.

Answer 2

The second one, the $_SESSION[...] = ... they just set the value inside your $_SESSION array and die() triggers this function:
session_write_close
(PHP 4 >= 4.0.4, PHP 5)
session_write_close — Write session data and end session
Report a bug Description
void session_write_close ( void ) End the current session and store
session data.
Session data is usually stored after your script terminated without
the need to call session_write_close(), but as session data is locked
to prevent concurrent writes only one script may operate on a session
at any time. When using framesets together with sessions you will
experience the frames loading one by one due to this locking. You can
reduce the time needed to load all the frames by ending the session as
soon as all changes to session variables are done.
http://nl.php.net/manual/en/function.session-write-close.php

Answer 3

So, I tried to test this by doing the following.
test_1.php
session_start();
$_SESSION['my_variable']=1;
sleep(20);
exit;
and
test_2.php
session_start();
var_dump($_SESSION);
Here are the cases I tested:
Executing test_1.php then quickly executing test_2.php.
Result: test_2.php froze until test_1.php finished sleeping.
Executing test_1.php, removing the PHPSESSID cookie and THEN
executing test_2.php.
Result: test_2.php executed right away with no freezing (test_1.php in the meantime was still sleeping) and printed an empty array.
Novice conclusion:
PHP opens a stream to the text file related to that session until the script has finished executing and then it "commits" the changes. (I'm not an experts, so my terms might not be accurate)
Edit: yet another useless test due to not consulting the documentation
but as session data is locked to prevent concurrent writes only one
script may operate on a session at any time

Answer 4

Sessions will be written in cookies or whatever storage you choose only after flushing the setter page. ex:
having pageA and pageB :
setting in pageA :
session["foo"] = "bar";
session["foo"] will be available to pageB only after:
leaving pageA to pageB
or
leaving pageA and refreshing pageB
that considered, the best definition IMHO is the second one.
Happy coding!