What can I do if my webhost doesn't support PHP 5.2?

The first thing you should do is contact their support representatives and politely ask when they will support PHP 5.2. Many web hosts may have plans to upgrade their systems that they haven't announced yet. If they don't, you can direct them to this web site to show that there is a need to upgrade to modern software versions. Give them some time; most web hosts won't immediately respond but will have to consider how they want to proceed.

If your web host says that they will definitely not be upgrading to PHP 5.2 but you depend on a project that requires it, the best thing you can do is find a new web host that will keep their software up to date.