You should explain more detailed, because there is no "php session" (what do you mean by that?), as PHP is either a module beyond Apache or is called by Apache via CGI. Anyway, there is always a parent Apache process which triggers the PHP execution.
There is no "pure" PHP Session in a webserver environment.
Also PHP scripts *should* execute very quickly. If a PHP Script takes large amount of time, it should not be executed in an Online Environment, but either on localhost only or as a batch job. So what is the reason for this problem?
P.S.: FInally i cannot resolve how someone can start more than one PHP process in his browser, because normally the browser (and client) whaits for the end of the execution, before triggering any other action.