PHP incorrect precision while rounding off number - php

Question

I need to round some numbers and get modifying value.
I've tried:
$Num=12.456;
$RoundDiff=$Num-round($Num,2);
$Num=round($Num,2);
echo $Num.'<br/>'.$RoundDiff;
but I've got:
12.46
-0.0040000000000013
while my expected result was:
12.46
-0.004
What's wrong?
How can I get what I need?

Answer 1

Try below code. It will give you the expected result. I have just rounded off the difference.
$Num = 12.456;
$RoundDiff = $Num-round($Num,2);
$Num = round($Num,2);
echo $Num.'<br/>'.round($RoundDiff,3);
CodePad
There is issue in precision of floating point values. Refer this article for reference - The PHP floating point precision is wrong by default
If you want exact precision you can use bcmath or gmp.
Because internally, computers use a format (binary floating-point)
that cannot accurately represent a number like 0.1, 0.2 or 0.3 at all.
When the code is compiled or interpreted, your “0.1” is already
rounded to the nearest number in that format, which results in a small
rounding error even before the calculation happens. — floating point
guide
Another Reference :
Given that the implicit precision of a (normal) IEEE 754 double
precision number is slightly less than 16 digits 3, this is a
serious overkill. Put another way, while the mantissa is composed of
52 bits plus 1 implicit bit, 100 decimal digits can carry up to
100*log2(10) =~ 332 bits of information, around 6 times more.
Given this, I propose changing the default precision to 17 (while the
precision is slightly less than 16, a 17th digit is necessary because
the first decimal digit carries little information when it is low). —
source
BCMATH : As requested in comments
$a = 12.456;
$b = round($a,2);
echo 'a ='.$a.'<br>';
echo 'b ='.$b.'<br>';
echo bcsub($a, $b, 3);