koshelevsergey

function groupBy(array $elements, callable $getUniqueKey) {
    $grouped = [];
    
    foreach ($elements as $element) {
        $uniqueKey = $getUniqueKey($element);
        $grouped[$uniqueKey][] = $element;
    }
    
    return $grouped;
}

$groupedByDate = groupBy($app, function($event) {
    return date("Y-m-d", strtotime($event['created_at']));
});