When you starting to get a grip on PHP, there are things that you should know, here I would descibe some of them, that might be useful,

1. OS
Afaik PHP can be installed on Windows, Linux, Macintosh, and Solaris.

2. Database
MySQL and PostgreSQL are the most popular database to be use with PHP.

3. WebServer
The most well-known web server for PHP is Apache and Nginx.

4. Application Framework
These are the most popular framework that so many people used, Zend Framework , CodeIgniter, CakePHP, Symfony, Laravel, Yii

5. PHP and OpenSource
PHP it self is an open source, in fact, so many app built with PHP is also open source i.e. WordPress, Drupal, Joomla, SugarCRM, ELGG, Dolphin, etc.

6. PHP and Tech Industry If you haven’t known this before, there is so many big company that used PHP for their business, you can take a look at Facebook, Yahoo, Flickr, WordPress, etc. Zend Technology is the company behind PHP, it’s like the commercial side of PHP.

7. The last but not least is of course the PHP it self
PHP is focusing on server-side scripting, but it can be use as Command line scripting and writing desktop application using PHP-GTK

A little bit history, PHP have two major version that commonly used, PHP4 and PHP. Basic OOP functionality was added in PHP 3 and improved in PHP 4. Object handling was completely rewritten for PHP 5, expanding the feature set and enhancing performance.