if($_POST['action'] == 'mark_arrived') {

    $trip_id = $_POST['trip_id'];

    // 1. GET CURRENT STOP
    $stop = $conn->query("
        SELECT * FROM trip_stops
        WHERE trip_id = $trip_id AND status = 'pending'
        ORDER BY stop_order ASC
        LIMIT 1
    ")->fetch_assoc();

    // 2. MARK DONE
    $conn->query("
        UPDATE trip_stops 
        SET status = 'done'
        WHERE id = ".$stop['id']."
    ");

    // 3. CHECK IF ANY LEFT
    $check = $conn->query("
        SELECT COUNT(*) as total 
        FROM trip_stops
        WHERE trip_id = $trip_id AND status = 'pending'
    ")->fetch_assoc();

    if($check['total'] == 0){
        $conn->query("
            UPDATE trips 
            SET status = 'completed', end_time = NOW()
            WHERE id = $trip_id
        ");
    }

    echo "OK";
}