This is something you'll probably have to take up with your hosting provider.
Try to figure out which PHP modules you need that are missing, and ask them to recompile their PHP to add those modules. If you don't know which modules, at least let them know what error messages you are getting so they can figure it out.
That is, assuming the problem really is that required modules aren't installed... It may be that the apps you are having problems with just can't cope with newer versions of PHP. It could even be as simple as a configuration setting (register_globals for example).
"Aren't working" is pretty vague, so I'm really just guessing here.
