PHP is an extremely popular, Open Source scripting language, most commonly used on webservers to produce dynamic pages. The name "PHP" is a recursive acronym for "PHP: Hypertext Preprocessor" and was initially created by Rasmus Lerdorf in 1994. As of today, the current version of PHP is version 5, with version 6 in the making.
As mentioned, PHP is mostly used on websites, which is what this tutorial will focus on. You may wish to set up your own test server, which is very easy to do, using the next couple of chapters to help you out. If you already have a server up and running, or at least access to a server running PHP 5, you can jump to one of the next sections, where we will start writing some PHP. You may go through all the chapters of this tutorial step by step, or you may jump between chapters to learn specific things - it's really up to you.
A basic knowledge of HTML and CSS is recommended when you're learning PHP. If you're completely new to HTML, you might wish to take a look at this excellent HTML5 tutorial as well as this fine CSS3 tutorial before proceeding.