In your opinion, why does Facebook use PHP as a programming language?

Simply to maintain the record of users in a database on server side which is the main feature of Hypertext processor(PHP) scripting language.Hope you understand my opinion.

It provides a lot of functionality with a very low learning curve. Run an installer, open Notepad, write a few lines of code and BAM, instant gratification

Facebook doesn’t use PHP for its core system, at Facebook, they uses C++ heavily on back end system. Apparently Facebook has many things built in PHP. Actually it’s not the real PHP, Facebook has written alternative version of PHP that is, HipHop’s HPHP’c which help to convert PHP code into C++ which shows that FB not using PHP, but by still using PHP. 

above article mentioned by :https://www.cuelogic.com/blog/how-facebook-is-not-using-php-by-still-using-php