sending three arrays over json

I have 3, one dimensional arrays, $PR, $AC, and $BC that I want to json_encode and send as one unit back to my browser app:
$this->output
 ->set_content_type('application/json')
 ->set_output(json_encode($data));
I thought I could combine them with $data=$BC+$AC+$PR but that is not working. On the client side I am just seeing $BC. Can I create a three d array and send it over the link?

I also want to send a checksum. Any way to create this checksum?

proof that an old dog can learn new tricks