用PHP round（）和14个十进制浮点数舍入不正确

Question

I have the age-old floating point rounding issue as many people before me have, but I can't seem to find a good solution for my situation. I'm doing some math operations in MySQL and returning the result to PHP as a float, then using round() to round the result. The problem I'm running into is with numbers like 10.50499999999977 which seem to trip up round() , rounding the number to 10.5 instead of the expected 10.51 .

Is there a consistent way to have PHP round a number like this correctly? I found a quick solution, but I'm afraid this wouldn't hold up in the long run:

echo round(10.50499999999977, 2, PHP_ROUND_HALF_UP); // 10.5
echo round(round(10.50499999999977, 5, PHP_ROUND_HALF_UP), 2); // 10.51

I'm especially worried that if I use this solution and try to round a number like 10.50444449 that this would end up rounding wrong (10.5 vs 10.51).

So is there a solution that I can use to get a consistently correct rounded number with PHP?

Answer 1

You're seeing expected behavior:

php>  echo round(10.50499999999977);
11
php > echo round(10.50499999999977, 1);
10.5
php > echo round(10.50499999999977, 2);
10.5
php > echo round(10.50499999999977, 3);
10.505
php > echo round(10.50499999999977, 4);
10.505
php > echo round(10.50499999999977, 6);
10.505
php > echo round(10.50499999999977, 7);
10.505

As per the docs:

PHP_ROUND_HALF_UP Round val up to precision decimal places away from zero, when it is half way there. Making 1.5 into 2 and -1.5 into -2.

...049999 isn't "half-way there". it's below 0.5, so php rounds down.

Answer 2

round() is doing exactly what you're asking:

echo round(10.50499999999977, 2, PHP_ROUND_HALF_UP); // 10.5

You're asking for rounding to two decimal places, rounding 'half' and above up.

Here the third decimal place and beyond is less than 'half' (0.00499999999777) so it's rounded down , giving 10.500. The trailing zeroes are dropped as they have no significance.