Why is Bcmath returning innacurate results

Question

Im having trouble getting bcmath to work with bitcoin based fractions on my server php 7.1 , ubuntu 18. Look at the following Code

bcscale(8);
$x1 = bcsub(0.04217 ,0.00007, 8);
$x2 = 0.04217 - 0.00007 ;
dd($x1 , $x2);

Result

"0.04217000"
0.0421

As you can see bcmath get return the first operand with some zeros added to it??. Any Ideas?

Answer 1

The manual is a little subtile but, the parameters are supposed to be strings. If you make them strings it will work.

bcscale(8);
$x1 = bcsub('0.04217' ,'0.00007', 8);
$x2 = 0.04217 - 0.00007 ;

echo 'x1 = '. $x1 . PHP_EOL;
echo 'x2 = '. $x2;

RESULT

x1 = 0.04210000
x2 = 0.0421

Also from the manual

Caution Passing values of type float to a BCMath function which expects a string as operand may not have the desired effect due to the way PHP converts float values to string, namely that the string may be in exponential notation (what is not supported by BCMath), and that the decimal separator is locale dependend (while BCMath always expects a decimal point).

As to the precision,

bcscale(8);
$x1 = bcsub('0.04217' ,'0.00007', 6);
//                                ^
$x2 = 0.04217 - 0.00007 ;

echo 'x1 = '. $x1 . PHP_EOL;
echo 'x2 = '. $x2;

RESULT

x1 = 0.042100
x2 = 0.0421

And

bcscale(8);
$x1 = bcsub('0.04217' ,'0.00007', 4);
//                                ^
$x2 = 0.04217 - 0.00007 ;

echo 'x1 = '. $x1 . PHP_EOL;
echo 'x2 = '. $x2;

RESULT

x1 = 0.0421
x2 = 0.0421