round() will sometimes return E notation when rounding a float when the amount is small enough - see https://bugs.php.net/bug.php?id=44223 . Apparently it's a feature.
To work around this "feature" when converting to a string, surround your round statement with an sprintf:
sprintf("%.10f", round( $amountToBeRounded, 10));