How does Apache process several requests at a time?

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http://serverfault.com – In a short question: If 10 requests hit Apache, does it process them one by one, so when R3 finishes, then it starts to run R4, or does it fire 10 processes/threads/whatever and are resolved simultaneously? Now some background: I have a PHP script that takes up to two minutes to do some processes. My question is: while a client is waiting for this 2 minutes, all the other clients requests are being processed? Or also waiting for this one to end? By the way, if there are simultaneous request, how can I handle them? Let's say put a limit on them. Or a limit on resources consumed. For instance (HowTos)