You can use Simple JSON for PHP. It sends the headers help you to forge the JSON.
It looks like :
<?php
// Include the json class
include('includes/json.php');
// Then create the PHP-Json Object to suits your needs
// Set a variable ; var name = {}
$Json = new json('var', 'name');
// Fire a callback ; callback({});
$Json = new json('callback', 'name');
// Just send a raw JSON ; {}
$Json = new json();
// Build data
$object = new stdClass();
$object->test = 'OK';
$arraytest = array('1','2','3');
$jsonOnly = '{"Hello" : "darling"}';
// Add some content
$Json->add('width', '565px');
$Json->add('You are logged IN');
$Json->add('An_Object', $object);
$Json->add("An_Array",$arraytest);
$Json->add("A_Json",$jsonOnly);
// Finally, send the JSON.
$Json->send();
?>