Question about apache-php concurrent process control

I am a programmer and new to php. I wonder what process control can php interpreter do for multithreading . I notice that through http request, php interpreter can execute 2 php programs simataneously, but will only sequentially queued and execute if I try to execute one php code in the same time. Can somebody tell me is it a problem by php design not to be execute code concurrently, or some apache/php configuration problem where I can fix? Thank you very much