Bcmul reporting 0

Question

I have a simple piece of code as below.

$amount = 447274.44882;
$rate = 0.00001;

echo floatNumber(bcmul($amount, $rate, 8), 8);

This outputs 0.00000000 when it should be 4.47274449. If I change the rate to 0.0001 then it outputs the correct number, anything higher than 4 decimals and it reports 0.

Am I doing something wrong or is this a known limitation or something? Seems quite a big one if that's the case.

Answer 1

If you cast 0.00001 to string using the default settings (and that's what will happen if you feed bcmul with floats since it expects strings) you'll get this:

var_dump( (string)0.00001 );

string(6) "1.0E-5"

It isn't clearly documented but bcmath functions apparently return cast to zero when faced to invalid input:

var_dump( bcadd('Hello', 'world!', 8) );
var_dump( bcadd('33', 'Foo', 8) );
var_dump( bcdiv('33', 'Foo', 8) );

string(10) "0.00000000"
string(11) "33.00000000"
Warning: bcdiv(): Division by zero
NULL

The whole idea of arbitrary precision libraries is to overcome the limitations of base 2 arithmetic and fixed size storage. Thus you'd need this:

var_dump( bcmul('447274.44882', '0.00001', 8) );

string(10) "4.47274448"

This is great to do math with 100-digit numbers but not particularly useful for simple rounding. In fact, the extension doesn't round at all—it merely truncates:

var_dump( bcmul('20.01', '1.444', 3) );
var_dump( bcmul('20.01', '1.444', 2) );
var_dump( bcmul('20.01', '1.444', 1) );
var_dump( bcmul('20.01', '1.444', 0) );

string(6) "28.894"
string(5) "28.89"
string(4) "28.8"
string(2) "28"