WordPress 3.2 will require PHP 5.2 or higher, as a result of which I’ve decided that my WordPress SEO plugin, currently still in beta, will also require PHP 5.2 and I will probably start having the same requirements for future version of my other plugins. This saves me time coding and testing against a version of PHP that is broken, slow and generally not always easy to work with.
It’s not that my plugin requires it per se right now to do what it does, although, knowing that for instance
json_encode is always there is a great improvement. It’s really the fact that it saves me testing time on older systems. As of the last release, because of that, my WordPress SEO plugin will automatically disable itself when it detects that you’re running a PHP version lower than 5.2, linking to this post.
Let me say this: if you’re hosting with a hosting company, and they still offer you PHP4 as a default, they’re daft and I wouldn’t trust them. I’ve written extensively about what I think about WordPress Hosting, it might be a good idea for you to review that and switch hosting to WestHost or VPS.net now if you encounter that issue.