[SOLVED] Is it safe to verify a form with the superglobal $

Is it safe to verify a form with the superglobal $_SESSION variable?

I have a form and when I submit it, I do an AJAX call to my server. On the server-side, I verify the informations by comparing them with some variables in the superglobal $_SESSION like below :

HeCanBuyIt = $ajaxData->priceProduct <= $_SESSION["user"]->moneyOfUser;

I am not sure if it is safe or not to do that (Can the user change the moneyOfUser variable in his session?).

I can also read the user from the database but it costs the time of a SELECT... I know it's not so slow but I prefer the fastest way.

Solution

All values in the $_SESSION variable are stored only on the server. The client is only given a session ID, which is stored in a cookie in their browser. There is no way for a user to view or manipulate the values in their $_SESSION unless you have explicitly coded that into your program.