It's now possible to use chatto on PHP 5.0.
I wrote the patch to bypass the native json function embedded in PHP5.2 with a custom JSON parser written in PHP.
To install the patch just extract the archive and copy the files into your Chatto installation directory.
Link to patch:
robertobifulco.it/Chatto/patches/Chatto-patchPHP5.7z
robertobifulco.it/Chatto/patches/Chatto-patchPHP5.zip