The large majority of the worlds websites run on PHP including Facebook because it was made for the web. It is no surprise therefore that WordPress is built using PHP.
PHP is technically a scripting language meaning that it does not necessarily have the same power and efficiency as languages such as Java. It is for this reason that you need to ensure your developers are experts in PHP, it is very easy to write code which works but is awfully inefficient.
With our many years of experience in PHP you can be confident we know everything that needs to be known about making the most of the language.