Better refactoring of map function

I'm currently refactoring my methods. I already eliminated long foreach and if states but now i'm struggling a bit with last finishing to have a nice map function.

My method looks like.

/**
 * @param \Illuminate\Support\Collection
 * @return array
 */
public static function convertTimesToChartData($times) {
    $clientTotal = '0';
    $arrayIndex = 0;
    $chartData = collect($times)->filter(function($item) {
        return !is_null($item->end_time);
    })->map(function($item) use(&$clientTotal, &$arrayIndex){
        $clients[$arrayIndex]['id'] = $item->client_id;
        $clients[$arrayIndex]['project_id'] = $item->project_id;
        $clients[$arrayIndex]['label'] = $item->company;
        $clients[$arrayIndex]['sec'] = $item->end_time->timestamp - $item->start_time->timestamp;
        $clientTotal += $item->end_time->timestamp - $item->start_time->timestamp;
        $arrayIndex++;
        return $clients;
    })->flatMap(function($item) { return $item; });


    return $chartData;
}

Here, i have 2 questions:

1. Is there a better way assign the array? When i'm trying to assigning directly in the return like

           return [
           [$arrayIndex]['id'] => $item->client_id,
           [$arrayIndex]['project_id'] => $item->project_id,
           [$arrayIndex]['label'] => $item->company,
           [$arrayIndex]['sec'] => $item->end_time->timestamp - $item->start_time->timestamp,
       ];
   
   

But then i get an undefined index error.

2. How is the best way to return a summarized array? cause i have several time entries for same client, so at the end, i just want the summarized seconds. It works on my example but i think there are better ways to do that. Specially because i need to define an $arrayIndex which i need to declare outside of the map function and reference to it. Don't think that this is the best way.

Thanks for any advice and help!

via mastercheef85