PHP originally reffered to as Personal Home Page, but later on renamed as Hypertext Preprocessor when the language was commonly being used on the web, at version 3.0.

PHP is a behind the scene scripting language that was originally designed for web development, to produce dynamic and interactive web pages.

It is generally used with HTML and runs on a web server. This web server needs to be configured to process PHP code and create web page content from it. PHP is Open Source and free to use, so it can be deployed on almost all web servers and near about on every operating system and platform.

PHP has evolved to include a command line interface capability and can also be used in standalone graphical applications. PHP supports many databases (MySQL, Informix, Oracle, Sybase, Solid, PostgreSQL, Generic ODBC, etc.)
Much of its syntax is borrowed from C, Java and Perl with a couple of unique PHP-specific features thrown in.

What You Should Already Know
PHP learning requires some basic knowledge of
HTML/XHTML
Javascript

Since using PHP for web development will require embedding it into HTML and some JavaScript may be used somewhere here and there. So its better to have some working knowledge of both. If you don’t have then you may learn them in the meantime.

MySQL

• MySQL is a database server
• MySQL is ideal for both small and large applications
• MySQL supports standard SQL
• MySQL compiles on a number of platforms
• MySQL is free to download and use

PHP + MySQL

PHP combined with MySQL are cross-platform (you can develop in Windows and serve on a Unix platform)