PHP is an open source scripting language (see What is PHP?) and it is a powerful programming language if you can master the language. There are lots of online reference to getting started with PHP, you can start by visiting the simple tutorial at official PHP website to get a feel of what PHP is and how you can build dynamic web pages with this popular programming language. It is recommended to get a good book with hands on examples if you are serious of mastering the language.
MySQL is an open source relational database management system (see What is MySQL?) that offer a robust database server for keeping your data, MySQL is popular choice among PHP developers and lots of well known web applications an entity are build using both, it is claim to be a much stable and affordable platform by most developers. MySQL follow the Structured Query Language standards, if you are new to both, perhaps you can start by going through a Simple tutorial at MySQL official website. A good book is a must to master the usage of MySQL and using PHP to interact with MySQL server for generating dynamic web pages.