Dynamic content web sites
In learning PHP and MySQL you need to understand that one of the greatest uses of these programs is in the form of Content Management System (CMS). As you can see, CMS is the main support of a dynamic web site. In fact, successful publishing sytems like Drupal and Wordpress utilize server side scripting and a database, in which the top choices are often PHP and MySQL.
But PHP and MySQL are not the only scripting languages and databases in the market. There are also others like Oracle and DB2 which sometimes offer better choice than the former. The reason why PHP and MySQL have become popular choices however is that PHP is more manageable while MySQL is a placed in a more established and secure Open Source database position.