How to fix Array error Undefined Offset in PHP

Concat the data that have same date but error on 3rd iteration. Concat the data that have same date but error on 3rd iteration.

 $summary = SummaryReport::with('accomplishments.employee')->with(['accomplishments' => function 
 ($query) use ($employee) {
        $query->where('emp_id', $employee->id);
        $query->orderBy('date', 'ASC');
 }])->find($id);

    $accomplishments = [];
    $date = "";

    for ($i = 0; $i < count($summary->accomplishments); $i++) {
        if ($summary->accomplishments[$i]["date"] == $date) {
            $accomplishments[$i - 1]['accomplishment'] = $accomplishments[$i - 1]['accomplishment'] . ', ' . $summary->accomplishments[$i]['accomplishment'];
            continue;
        }
        array_push($accomplishments, $summary->accomplishments[$i]);
        $date = $summary->accomplishments[$i]['date'];
    }

Accomplishments of a particular employee from a particular SummaryReport, grouped by the date

After your query do the following

$acc_by_date = $summary->accomplishments->groupBy('date');

$result = [];
foreach($acc_by_date as $date => $accs){
   $result[$date] = [
       'accomplishments' => $accs->pluck('accomplishment')->reduce(
           function ($carry, $item) {
              return $carry . ", " .  $item;
       }),
       'remarks' => $accs->pluck('remarks')->reduce(
           function ($carry, $item) {
              return $carry . ", " .  $item;
       }),
       'remarks_as_array' => $accs->pluck('remarks') 
   ];
}