The most popular open-source Relational Database Management System (RDBMS), it is ACID compliant, has high availability, and is accessed using the SQL language. Available on all platforms, there are API's to connect it to most languages, and compliments PHP very well.
I started using MySQL in 2003, before doing my university placement. It has been my go-to database ever since. Over the years I have developed many projects that use MySQL, and mastered some of its more advanced features such as aggregate queries, sub-queries, unions, indexes, and execution plan optimisation.