title: Do PHP pages on a server run simultaneously?
body: <p>This probably seems like a very simple question, and I would probably know if I had a more in depth knowledge of computer processes and the like, but anyway..</p>
  
  <p>If two people request the same page from my server, is the PHP page processed once for the first person, and then a second time for the second person, or might these run along side each other at the same time?</p>
  
  <p>Take this as an example. I have one stock Item left in my PHP driven online shop. A user adds this to their cart. Php script 1) checks to see if it is in stock, Yup, its in stock, so it 2)reserves it for him.</p>
  
  <p>What If, in between checking if its in stock and reserving it, the same PHP page was loading for someone else, and just after user A checked if it was in stock, so did user B, before user A got a chance to reserve it, so they both end up reserving it!</p>
  
  <p>Sorry if this seems silly, can't seem to find an answer on it, which is it?</p>