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?
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); 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, $b, 3); 
bcsub($Num, round($Num,2), 20);
1/3is not the same as0.33333no matter how many3we put after the decimal dot. In practice we usually tend to think they are the same thing but the latter is an approximation of the former. Computers work the same, only the numbers that can be written asm/nwherenis a power of2can be represented exactly. All the other are approximated. It all depends how much of the error you choose to ignore.