Well I think this issue is not that simple, since the OP is using a third party commercial script, something like vBulletin. I am sure the OP did not write any of these functions, which is why hes so confused. I checked the website of PHP Probid, and it confirms my assumption.
Its not recommended for client users to directly alter a third party library code, unless you know what you are doing. This makes future maintenance much more difficult, and some softwares may offer upgrade service which will revert the changes made by client users when they upgrade. Even I refrain from editing third party library source code, unless its not maintained by the original coder anymore, or that I decide to take over its development myself.
Did you report this to the company that sold you this software? What did they say? Maybe they explicitly make it clear that their software cannot run on PHP 5.5+? I think its worth reporting the issue to your software provider, and ask them for a solution. Perhaps their next version will use a different function for password hashing.