Internet Explorer 8+, Firefox 3.5+, Safari 4+, Chrome, and Opera 10+ support native JSON parsing.
If you are thinking using one older browser then you have to include the json2.js file in the head of the html document.
The json_encode function is supported by PHP >= 5.2.0.
So, if your PHP version is lower that that you have to include the JSON.php in the beggining of your php file.
I wanna thank to Cory LaViska of http://abeautifulsite.net who come with a very easy solution to this and to Michal Migurski for authoring this fantasic PHP class.