Using a unix crontab will solve the problem.
It s not really suited for php script but you can use it anyway: either by having php as an executable (so you can launch it as any other scripts) or by using wget to request the wanted page to be loaded by apache (in case your php is not an executable but an apache's module).
Among other source of information about crontables, here is one that looks nice:
http://kb.indiana.edu/data/afiz.html