Most people use PHP through Apache, and most of the remainder use it through Nginx. Regardless of which one you choose, make sure you're on the latest version – certainly nothing lower than 2.4 for Apache and 1.8 for Nginx.
You should also be aware there's a whole other world of optimization strategies for web servers, although it gets more complicated because many choices depend on your exact site configuration. For example, on my own sites that demand maximum responsiveness across many connections, I nearly always disable HTTP keep alives, because they just use up an extra Apache connection to save on page load time.
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