Question

If the PHP Engine is already in the middle of executing a script on the server what would happen to other simultaneous browser requests to the same script?

• Will the requests be queued?
• Will they be ignored?
• Will each request have its own script
  instance?
• Any other possibility?

Answer 1

The server, depending on its configuration, can generally serve hundreds of requests at the same time — if using Apache, the MaxClients configuration option is the one saying :

The MaxClients directive sets the
limit on the number of simultaneous
requests that will be served.
Any
connection attempts over the
MaxClients limit will normally be
queued, up to a number based on the
ListenBacklog directive.
Once a child
process is freed at the end of a
different request, the connection will
then be serviced.

The fact that two clients request the same page is not a problem.

So :

Will the requests be queued?

No ; except if :

• there is some lock somewhere — which can happen, for instance, if the two requests come from the same client, and you are using file-based sessions in PHP : while a script is being executed, the session is “locked”, which means the server/client will have to wait until the first request is finished (and the file unlocked) to be able to use the file to open the session for the second user.
• the requests come from the same client AND the same browser; most browsers will queue the requests in this case, even when there is nothing server-side producing this behaviour.
• there are more than MaxClients currently active processes — see the quote from Apache’s manual just before.

Will they be ignored?

No : this would mean only one user can use a website at the same time ; this would not be quite nice, would it ?

If it was the case, I could not post this answer, if you where hitting F5 at the same moment to see if someone answered !

(Well, SO is not in PHP, but the principles are the same)

Any other possibility?

Yes ^^

edit after you edited the OP and the comment :

Will each request have its own script
instance?

There is no such thing as “script instance” : put simply, what’s happening where a request to a script is made is :

• the webserver forks another process to handle the request (often, for performance reasons, those forks are made in advance, but this changes nothing)
• the process reads the PHP script from disk
  • several processes can do this at the same time : there is no locking on file reading
  • the file is loaded into memory ; in a distinct memory block for each process
• the PHP file in memory is “compiled” to opcodes — still in memory
• those opcodes are executed — still from the block of memory that belongs to the process answering your request

Really, you can have two users sending a request to the same PHP script (or to distinct PHP scripts that all include the same PHP file) ; that’s definitly not a problem, or none of the website I ever worked on would work !