PHP requests one by one or simultaneously

Question

I have got some questions about PHP and how requests work under the hood.

1) Let's say I wrote my PHP application and upload it to server. Now there's a function that I wrote and if user goes to that route which executes that function, something happens.

Question is: if one user makes the request and another user also makes the request, does the second user have to wait until first user's request is done? (by saying request is done, I mean until the function I wrote gets executed through the end). Is this the correct guess or it doesn't matter which function gets executed. Until the request isn't done, the second request never starts?

2) I have my PHP application. Imagine two persons make the request at the same time which writes data to the database (not writing, but updating). Let's say I used load balancers. if one user makes request to balancer1 and another user makes request to balancer2, what I want to do is if first user's call updates the database, second user's request has to stop immediately (it shouldn't be updated).

The scenario is that I have jwt token in my database which is used to do requests on third party tool. it has expiration 1 hour. Let's say 1 hour has passed. If one user makes the call to update the token and in the way second user also makes the call to update the token, what will happen is second user will update the token and first user's token will be invalid, which is bad.

Answer 1

PHP can handle multiple requests at the same time, but requests from the same user will be processed one by one if the user's PHP session is locked by the first request. Second request will be proceeded when session would be closed.

For example, if you run a PHP script with sleep(30) in one browser tab:

<?
session_start();

sleep(30);

And another script in another tab:

<?
session_start();

echo 'hello';

The second script won't be executed until the first one is finished.

It's important to note this behavior because sessions are used in almost every app.