Unable to delete mongodb document with PHP

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PHP

MongoDB

I am trying to delete a record from mongodb database using php by passing mongodb ID. The code executes fine but it's not deleting the specified document, does anyone see any problem with the code below ??

 $postData = json_decode(file_get_contents("php://input"), true);

    $conn = new MongoClient();
    $db = $conn->selectDB('employees');
    $collection = new MongoCollection($db, 'employees');
    $emp = array('id' => new MongoID($postData['$id']), true);

    try {
        $collection->remove($emp, array('justOne' => true));
    }
    catch (MongoCursorException $e) {
        die(json_encode(array('message' => $e->getMessage())));
    }

    echo ('The employee has been deleted successfully');
}

What I have tried:

I have tried Stackoverflow but people there don't seem to take interest in this question.