Get value from JSON array in PHP

First you have to decode your json data

$json = json_decode($data[0]['json']);

Then you can access your AfterParticipationHeader

$json->Canvas[0]->MainObjects->{"After Participation"}->afterParticipationHeader

you can convert a valid JSON string to a PHP variable with json_decode(). Note the second parameter to get an assoc array instead of the less usefull stdClass.

$jsonData = json_decode($data[0]['json'], true);
$header = $jsonData['Canvas']['MainObjects']['After Participation']['afterParticipationHeader'];

You can decode the JSON via the json_decode function:

$json = json_decode($data[0]['json']);

Then, you'll have arrays (in the same structure) with your data.