PHP is one of the most widely-used server-side scripting languages on the internet today. It is fast, secure and, best of all, open source (free). It can be used to create dynamic content, forms and form processors, connect to databases, and many other things.
The beauty of PHP is found in its simplicity: it can be inserted into any html page and is simple to learn and understand. When scripts are coded properly, they do not take a lot of resources to run fast, leaving the resources for other processes that need it more. PHP also uses a modular model for its functions and extensions, which can be enabled or disabled through the php.ini file, some of which are security measures to protect against malicious attacks.
PHP also offers a wide range of connection abilities: it will connect to graphics processors, XML parsers, and encryption engines, as well as databases: MySQL, MsSQL, Oracle, etc. Not to mention, it is usable on any hosting platform: Apache, IIS, Roxen, THTTPD, and AOLServer.
The real advantage to using PHP is the community. The PHP Community has a wide range of developers that participate in it, from the beginner to the developers that actually code the language, most of which are more than happy to help with any problems that crop up in coding. If a script that performs a particular function is needed, if the community is checked, chances are pretty good something similar will be found that only needs a little tweaking to make it work.