$j=0;

for ($i=0; $i<count($data); $i++)
{
      if ($data[$i]['action'] === 'reply_staff')
      {
           $reply[$j]['staff'] = $data[$i]['action_date'];
           $j++;
      }

    if ($data[$i]['action'] === 'reply_user')
    {
          $reply[$j]['user'] = $data[$i]['action_date'];
          $j++;
     }

}