在尝试写入数据库之前，always 验证您输入的值：

$fValid = true;
if ( !isset( $payment->status ) ) {
    echo \'Error: Payment status is not set\';
    $fValid = false;
}
if ( !isset( $order_id )  ) {
    echo \'Error: Order ID is not set\';
    $fValid = false;
}

if ( $fValid ) {
    // Update database
    $fSuccess = $wpdb->update( $wpdb->prefix . \'mollie_transactions\',
        array(
            \'status\'            => $payment->status
        ),
        array( 
            \'transaction_id\'    => $order_id
        )
    );

    echo sprintf( \'Update %s where Payment Status was %s and Transaction ID was %s\', $fSuccess? \'Succeeded\':\'Failed\', $payment->status, $order_id );
}