PHP and MySQL
PHP is one of the popular server-side scripting languages designed for web development. Today, a big segment of the "server side website development industry" is dominated by PHP. Besides, many popular sites like Wikipedia and Facebook as well as CMSs like WordPress, Magento, Drupal, and Joomla are built on PHP.
It acts as the base to develop CMS framework. So, to become a master in CMS frameworks like Wordpress, Joomla, Magento, it is highly necessary to be a master in PHP.