In my country, every good web hosting company offering me the choice of PHP version up to 5.5 or 5.6 now. In Control Panel with a few clicks I can upgrade or downgrade PHP version anytime I want and the effect is immediate. It became standard in a last year. I do not know how it is in other countries, but this is my experience.
Do not think only on functions. Think also on security issues. There is only one (not very good) reason why to use obsolete PHP versions. If you have old code which works and you do not want to touch it. One of my friends have on web hosting PHP 4.4, because the code of his application was wrote almost 10 years ago. But if you are making something new, you should not use obsolete PHP versions. In a few years, the PHP 5.6 will be the standard almost everywhere. And where not, those places should be avoided.
