PHP, as a program language, has its only life cycle.
The main three version of PHP is 5.3, 5.4, and 5.5.
PHP5.3 will be supported until July, 2014. It is still the main default version in CentOS.
PHP5.4 will be supported until March, 2015
PHP 5.5 will be supported until June, 2016.
Each version of PHP has three years life from the release date.
I have a performance test with real forum environment. It is running based on phpBB 3.0.11 forum software.
I am doing research and testing. I will post the results later.
p.s. PHP 5.6 is in beta now. Official release day is not confirmed.