使用array_multisort按组合值排序

Using PHP version 5.6, MySQL 5.7.

I have been using an array_multisort to sort rows by column. This has been working well sorting by 'apps', followed by 'subapps'. What I wish to do now is combine the values of 'apps' and 'subapps' to sort the column by a new value 'allapps', which would be the sum of 'apps' and 'subapps'.

// GET PLAYER INFO
while($data = $get_players->fetch_assoc())      
    {       
    // PLAYER ID
    $idt[$i] = $data['id'];

    // DETERMINE HOW TO DISPLAY NAME
    if($data['firstname'] == '')
        { $names[$i] = $data['lastname']; } 
    else
        { $names[$i] = $data['firstname'] . ' ' . $data['lastname']; }

    $i++;
    }

mysqli_free_result($get_players);

// COUNT APPEARANCES
$i=0;
while($data = $get_apps->fetch_assoc())
    {
    $apps[$i] = $data['apps'];
    $i++;
    }
mysqli_free_result($get_apps);

// COUNT SUBSTITUTE APPEARANCES
$i=0;
while($data = $get_subapps->fetch_assoc())
    {
    $subapps[$i] = $data['subapps'];
    $i++;
    }
mysqli_free_result($get_subapps);

Rather simplistically, this is what I've tried, which doesn't seem to work:

$allapps = $apps + $subapps;  

And below is the array_multisort:

array_multisort($allapps, SORT_DESC, SORT_NUMERIC, $apps, $subapps, SORT_DESC, SORT_NUMERIC, $names, $idt);

At the moment, it bypasses $allapps and just continues to sort by $apps, and then $subapps. How best should I combine the values of $apps and $subapps to sort by the combined value of $allapps?

Thank you in advance for any advice.