The api version you're using has been deprecated https://dev.twitter.com/docs/api/1/get/statuses/mentions
Please consider moving to use of 1.1 version of Twitter API: https://dev.twitter.com/docs/api/1.1/get/statuses/mentions_timeline
You might want to use the following instruction instead:
$json['mentions'] = $connection->get('/1.1/statuses/mentions');