Optimizing Apache and PHP

Apache is a highly configurable piece of software. It has a lot of features, but each one comes at a price. Tuning Apache is partially an exercise in proper allocation of resources, and involves stripping down the configuration to only what's needed.
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Apache is modular in that you can add and remove features easily. Multi-Processing Modules (MPMs) provide this modular functionality at the core of Apache -- managing the network connections and dispatching the requests. MPMs let you use threads or even move Apache to a different operating system.