Two pure-php implementations of the json protocol that might be of use, e.g. for php 4 installs where adding extensions is not an option or because of extra functionality can be found here:
http://sourceforge.net/projects/phpxmlrpc (look for the extras package)
http://mike.teczno.com/json.html
Of course, performance is abysmal compared to the php extension, written in highly optimized C code...