It it written using Symfony which is an extremely solid enterprise framework modeled on Java Spring.
PHP powers the majority of the web still and also has one of the largest development communities, proof of this usefulness is how many contributions have been made in the last few days: https://codeberg.org/Kbin/kbin-core/pulls
In the long run it will probably be a great advantage to Kbin to be written in PHP and for anyone who hasn't seen the latest versions of PHP go look at the source code and have a look at all the recent improvements to the language.