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.
PHP 5.4 with OPcache
PHP 5.5 with OPcache
If possible, I will recommend to use the latest stable version of PHP, PHP 5.5. But in the realistic world, I can not do that, because some software I use doesn’t support it. I have to use the old version PHP5.3.
And the problem bothers me is how to use multiple PHP version in same VPS, or even in same website.
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.