PHP and MySQL are most popular open source technologies in this time. PHP is one of the best open source scripting language that used to develop website and web applications.
My SQL is most powerful and popular open source database server built on RDBMS (Relation database management system) and capable to manage and handling large database.
When combined together, these technologies can provide you most powerful platform. And Expert developer can build capable and powerful web application.
PHP and MySQL are referred as a development tool.
These are open sources means they are free development tools and there is large community of programmers who make improvements and are continuously adding features.