Php backend fundamental threading -

i have php-backend. lets 2 users send rpc server. entrypoint service.php. service.php contains class service functions. service.php calls function class/object service. write or, cause i'm not sure how structure php-backends. leads question.

questions:

do rpc resolved (non-threading): rpc user1 done. rpc user2 done.

or (threading): rpc user1 being processed. rpc user2 being processed while rpc user1 still processed.

so can if use static variables , objects, rpc user2 changes static-varaibles while rpc user1 being processed?