I've been hunting the internet for a good Json Pretty Print for PHP when I finally discovered one right under my nose.
The purpose was to output Json in a human readable form, making it easier to see what data is being transferred through one of my projects.
PHP does not yet have its own implementation of Pretty Printing inside its Json system yet (as of PHP 5.3.x), however there is some intriguing constants inside the source code (look for JSON_PRETTY_PRINT, which can now also be found in the json_encode manual page).
Meanwhile Zend Framework seems to have this solved through its Zend_Json class. With Zend Framework being open source you can easily get hold of the source code to translate it to any project and language you need, as long as your environment supports Perl compatible regular expressions (preg_split).
It is not quite perfect but it does the job.
I currently have an exporter storing full user accounts in a Json object saved as a file, part of this data is a column in the database which stores Json itself, it seems the Zend_Json::prettyPrint function will also go ahead and pretty this up also, not quite expected but cool none the less!