although the Json is more widely acceptable serialization/deserialization scheme than WDDX, but the decoding speed with Chinese character in WDDX in PHP is much faster than Json; the same several MB serialized data file can deserialize in only ~80ms and for Json is 800ms, 10 times difference !!!
For those of you who wants to use JSON with PHP4 to process russian KOI8-r text, please read the following article: http://yozhek10nozhek.livejournal.com/5842.html (in russian)
Maybe it is neither completed yet nor professionally-written, but it doesn't aim to be some kind of full-featured manual.
Note, that you must have iconv extension loaded to use it; json extension is not needed, I use Services_JSON (http://pear.php.net/pepr/pepr-proposal-show.php?id=198) implementation.
The JSON function is basically a more widely accepted variation on the WDDX principle:
http://us2.php.net/manual/en/ref.wddx.php
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...