If you're building a web application and have high hopes that it will be used by lots of people, one of the most important things to do before you launch it is to test how it functions under extreme conditions.
PHP is installed on more than 20m websites, including some of the most popular websites in the world.
It's easy to install, easy to work with and when put in the hands of a competent developer can achieve the performance and scalability that highly-popular websites and demanding web applications require.