What Is PHP?

PHP is a widely-used general-purpose scripting language that is especially suited for Web development and can be embedded into HTML.

The endless possibilities of the PHP scripting language and a great community of users has made it one of the most popular open-source languages. For all you people living outside the UNIX world, Open Source means it doesn?t cost anything. You can use it as much as you want and where you want, and nobody will ever charge you thousands of dollars for licenses and support. Even though it was originally conceived as a set of macros to help coders maintain personal home pages, its name grew a lot more from its purpose. Since then, PHP?s capabilities have been extended, taking it beyond a set of utilities to a full-featured programming language, capable of managing huge database-driven online environments.

PHP stands for PHP: Hypertext Preprocessor
PHP is a server-side scripting language, like ASP
PHP scripts are executed on the server
PHP supports many databases (MySQL, Informix, Oracle, Sybase, Solid, PostgreSQL, Generic ODBC, etc.)
PHP is an open source software (OSS)
PHP is free to download and use

A PHP file may contain text, HTML tags and scripts. Scripts in a PHP file are executed on the server.

That's a mouthful, but if we break the definition down into smaller pieces, it is easier to understand.
server-side: This means that PHP scripts execute on the Web server, not within the browser on your local machine.
cross-platform: Cross-platform means that PHP scripts can run on many different operating systems and Web servers. PHP is available for the two most popular Web server configurations (IIS running on Windows NT and Apache running on UNIX).
HTML embedded scripting language: This means that PHP statements and commands are actually embedded in your HTML documents. When the Web server sees the PHP statements in the Web page, the server executes the statements and sends the resulting output along with the rest of the HTML. PHP commands are parsed by the server much like Active Server Pages or Cold Fusion tags.