PHP - is a scripting language originally designed for producing dynamic web pages. It has evolved to include a command line interface capability and can be used in standalone graphical applications.
PHP is a widely-used general-purpose scripting language that is especially suited for web development and can be embedded into HTML. It generally runs on a web server, taking PHP code as its input and creating web pages as output. It can be deployed on most web servers and on almost every operating system and platform free of charge.PHP is installed on more than 20 million websites and 1 million web servers
MySQL - is a relational database management system (RDBMS)
which has more than 11 million installations.The program runs as a server providing multi-user access to a number of databases.
MySQL is popular for web applications and acts as the database component of the LAMP, BAMP, MAMP, SAMP, and WAMP platforms (Linux/BSD/Mac/(Open)Solaris/Windows-Apache-MySQL-PHP/Perl/Python), and for open-source bug tracking tools like Bugzilla. Its popularity for use with web applications is closely tied to the popularity of PHP, which is often combined with MySQL. Several high-traffic web sites (including Flickr, Facebook, Wikipedia, Google, Nokia and YouTube) use MySQL for its data storage and logging of user data.