The recent release of php 5.2.0, which includes by default an extension for converting php values from and to the JSON format, is a good occasion for comparing the different existing php libraries which aim to provide the same capabilities.
Here you can find a detailed analysis of the features, performances and encoding/decoding capabilities of 4 of the main implementations: 3 “pure php” libraries and the native extension.
I have to warn readers: I personally maintain one of the four libs in exam.
Any correction, criticism or feedback is welcome.

Edit 2007/04/25 - an update version of the comparison has been posted here