Most people start with PHP by asking what the acronym stands for, well PHP stands for PHP Hypertext Preprocessor, which I guess is a bit of a developer's joke and means the definition is recursive.
The next question is why have I put PHP under Software Development and not Advanced Stuff like Python. Well, in my mind Python is a good scripting language and that is generally how I use it, sure you can build websites with it but I don't. However PHP is much more of a website software development language, at least in my mind, for one thing Drupal is built in PHP.
A good place to start with PHP is learning about the "types" that it has and how they work, so PHP: Types - Manual is a good place to do this.