PHP is a popular web scripting language used to add interactivity to a website. PHP is available for most operating systems, but is most widely used on Linux and Unix platforms, where it is the dominant web scripting language. It resembles Microsoft's Active Server Pages (ASP) on the Windows platform.
PHP enabled websites are able to interface with a wide variety of relational databases including Microsoft SQL Server and Oracle, although the open source MySQL is by far the most popular choice of database.
PHP allows scripting code to be placed within the HTML source of a web page, allowing a wide range of functionality to be added to a page. Unfortunately this can lead to application source code that can be difficult to maintain due to the interspersal of HTML with business logic. Microsoft have addressed this in the replacement for ASP - ASP.NET by allowing HTML markup and code to be separated into different files. A PHP software documentation utility such as one of those listed here can assist with making sense of PHP application source code.
Further information about PHP is available at http://www.php.net/.