Lorna Mitchell has taken a look at using JSON in PHP applications in a new post to her blog. More specifically, she looks at how to get it working and a downfall or two that comes with it.
She gives an example of the translation between a PHP array (of subarrays) out into a JSON message complete with grouping. She does mention one problem that using this data format has, though - the lack of typing information that comes along with the message details. That can mean the difference between an array being pushed in on the sending side and an object (from json_decode) coming out the other side. As noted in the comments, though, there's a second parameter for json_decode that tells it to force the output as an array instead (along with a mention of another helpful constant in PHP 5.3 - JSON_FORCE_OBJECT).