MongoDB v PHP pomocí agregace k seskupení podle _id nefunguje null

Použijte MongoRegex objekt:

$search = "apc";
$where = array('Store.Title' => array('$regex' => new MongoRegex("/^$search/gi")));
// $where = array('Store.Title' => array('$regex' => new MongoRegex("/".$search."/gi")));

[
    [ '$match' => $where ],
    [
        '$group' => [
            '_id' => 'null',
            'count' => [ '$sum' => 1 ]
        ]
    ]
]