What is PHP?
PHP (acronym for: PHP Hypertext Preprocessor), is a server-side embedded scripting language. This means that it works within an HTML document to confer to it the capacity of generating content on demand.

Why use PHP?
HTML and DHTML simply design the structure of the page content, and JavaScript allows one to do some bit of interaction at the client side. However, the Internet are being used for dynamic applications, which incorporate databases. This is where PHP comes in. By running PHP programs on the server, you can create very powerful applications that interact with a database and generate content dynamically.

You may be asking, "But, why PHP? There are so many other options like ASP, Cold Fusion, Perl, Java, Python and other good scripts.", and the answer will be: simplicity, speed, security & an almost natural way of using databases and platform independence.