PHP programming language has been popular among website developers for building small to complex website application, unlike other proprietary programming languages that requires paid licenses, PHP is released under Open Source License which is part of the reason for the growing popularity since it was introduced to the internet. PHP Hosting is commonly offered among linux based web hosting providers due to the large demand from php website developers, a much cheaper cost to maintain and reliable perfomance on Linux based web servers.
Official website for PHP : PHP Official website
MySQL is a database management system and has been tightly link to PHP programming language, web developers uses PHP to write codes to interact with the MySQL database, this allow building complex database driven website application. MySQL is also released under Open Source License
Official website for MySQL : MySQL Official website
This PHP MySQL Tutorials section comprises of some general knowledge of php and mysql, where to obtain more detail information on php programming language and mysql database.