Facebook uses PHP 5.
PHP
In terminal use the following command. php -ver
There are millions of websites that use PHP, some notable ones are Facebook, Digg, Wikipedia and many more.
The next version is PHP 6.
The lastest stable versions of PHP (as of March 3, 2012): PHP 5.4: Version 5.4.1 PHP 5.3: Version 5.3.11
Use PHP 5.4 version latest library .
Many jobs can use PHP, a lot of gaming companies around the United States are looking for PHP programmers. Zynga may or may not still be looking for it, and Facebook as well.
As of 19 December 2017, the latest stable version of PHP is 7.2.0.
Any version released before the current stable release is considered an old version of PHP.
PHP++ is an object-oriented version of the PHP programming language. ++ is used in programming to increment a variable by one so it means an improved version of PHP.
Primarily it is applied for creating web applications and websites which deal with client-server model. Use mysql db with php and you could build a three tire architecture. Couple of big website like Facebook do use php.
This is a very broad question, many web design companies will make sites in PHP. Facebook for instance is made in PHP, although the have modified it a bit for their massive traffic!