If you are using a control panel like cpanel, I believe its possible to set specific versions for a site.
I have bluehost. Under PHPconfig in the cpanel I am able to select which php version I want. It is currently set to PHP 5.6 (screen shot below). Please forgive me, I really don't know too much about the technical side of things. So I don't understand why my vbulletin is currently using PHP 5.2.17 (according to "View PHP info" in vb admincp) and not PHP 5.6. And I am concerned about upgrading vbulletin to 4.2.3, which requires PHP 5.4 or higher, and it being stuck at PHP 5.2.17 and the updated files not working because it isn't compatible.
If I went ahead and upgraded vbulletin to 4.2.3 would it use PHP 5.6? or would it stay at PHP 5.2.17?