The Dzone.com site has a recently posted tutorial helping you understand the php.ini, the main configuration file PHP uses to set up its own internal features and settings.

our php.ini file provides a considerable amount of power over the behavior of your PHP application ecosystem. Let’s jump into some of the most common declaratives and discuss how they impact your application performance and behavior. I won’t go into an explanation of each setting that is available, but I’ll cover the fundamental options that you should be aware of. Please keep in mind that changing any of the settings on in your php.ini can and may very well change the behavior of your application, whether positive or unfavorable. Please use caution when adjusting your settings, consult with your team, do your research, understand the implications, and, of course, test, test, and test again before deploying anything into production!

He starts with a brief introduction to what the php.ini file is and how you can use the phpinfo function to find the current settings in HTML form (on the command line it's "php -i"). He then goes through and covers some of the basics from the standpoint of security, memory handling and some general settings.