For example, Apache doesn’t serve up just HTML files—it handles a wide range of files, from images and Flash files to MP3 audio files, RSS (Really Simple Syndication) feeds, and so on. To do this, each element a web client encounters in an HTML page is also requested from the server, which then serves it up.
But these objects don’t have to be static files such as GIF images. They can all be generated by programs such as PHP scripts. That’s right: PHP can even create images and other files for you, either on the fly or in advance to serve up later.
To do this, you normally have modules either precompiled into Apache or PHP or called up at runtime. One such module is the GD library (short for Graphics Draw), which PHP uses to create and handle graphics.
Apache also supports a huge range of modules of its own. In addition to the PHP module, the most important for your purposes as a web programmer are the modules that handle security. Other examples are the Rewrite module, which enables the web server to handle a varying range of URL types and rewrite them to its own internal requirements, and the Proxy module, which you can use to serve up often-requested pages from a cache to ease the load on the server.
Later in the book, you’ll see how to actually use some of these modules to enhance the features provided by the three core technologies.