Parse JSON Object Array Data With PHP

Question

Why does ac return NULL here? I've verified with var_dump it has values

$data = array(
    'ac' => $cl->l->ac->dn,
    );

    foreach ($data as $key => $value) {
        echo "The $key is: ". json_encode($value);
        echo "<br>";
    }

This is the format it's in

  "ac": [
    {
      object (Category)
    }
  ]