PHP  is a widely-used general-purpose open source server-side scripting language that is especially suited for Web development when embedded into the HTML of web pages. Php scripts are embedded into HTML and are a great way to connect a remote database to your local web page. MySQL is the most commonly used database that works well in combination with php. The combination of php and database powers many popular forms of website such as forums, Wikis and Social Networking sites. Php is Open Source and Free.

One of the great things about php is it's ease of use, all you need to start to experience php is a text editor such as Notepad. However it is best to download and install WAMP to create and edit php. That way you can have a full on server on home (local) computer.

How HTML Works
When your  browser requests a web page the Apache server on the remote computer responds to that request by finding that page and sending it back to your browser.

How php Works
Web pages that have php in them have a *.php extension. Apache scans these pages for php script. When it finds php code embedded in the requested HTML page its sends that script to the php phaser. The php phaser is actually an interpreter. The php phaser scans the HTML for any php scripts and runs them. The resulting product that the php interpreter creates is embedded back into the web page in place of the php script. Apache now sends that page to your local computer.