PHP is most used programming script. The main reasons of popularity of PHP were that it can be run on any OS platform (not like ASP), you can find lot of support and much more.
Today most of the web programs run on PHP like most popular blog software WordPress is created on PHP, popular forum scripts like vBulleting and phpBB are created on PHP there are endless list of popular scripts which are created on PHP.
You can find lots of free tutorials on PHP. You can find PHP tutorials which will teach you PHP from beginning. Here I have listed some PHP tutorials which will teach you some interesting and useful things for your website.
5 Free PHP Tutorials for your Website