The great thing about PHP is that it is present in some form on nearly all servers. If you’re careful to avoid version 5 features then you can pretty much expect your code to run wherever.

For a recent prototype I needed JSON (which wasn’t introduced to PHP until version 5) on a 4.3 system. Being a relentless do-it-yourself’er I have written a tiny JSON functionality “library” for older versions of PHP.

It’s about 5k of mostly comments. Available under MIT licence from Google code. Have fun.