MySQL is one of the three major database servers that have shaped the history of the data collection of the web over the last 20 years. MySQL is the database server that prefers PHP as a language that you ask him to deliver results on the Web. The perfect relationship between PHP and MySQL has meant that others would pass mostly Database Server in the background or belong to different schools of thought.
Using MySQL server for custom projects practically I can remember. I find it of course in all projects that rest their foundations in CMS like WordPress or Drupal. MySQL is not only perhaps the best of the database server, but is also free and this part of the development WEB has an intrinsic value that goes far beyond the commercial assessment or simply tied to costs.