Here is a easier to read version of the test code (I also added one more):

$a = 70687.465 + 8.930;
$b = 70696.395;
$c = 70687.464 + 8.936;
$d = 70687.464 + 8.931;
printf("A: %s %s %.9f\n", $a, money_format("%.2n", $a), $a);
printf("B: %s %s %.9f\n", $b, money_format("%.2n", $b), $b);
printf("C: %s %s %.9f\n", $c, money_format("%.2n", $c), $c);
printf("D: %s %s %.9f\n", $d, money_format("%.2n", $d), $d);

Output:
A: 70696.395 70696.39 70696.395000000
B: 70696.395 70696.40 70696.395000000
C: 70696.4 70696.40 70696.400000000
D: 70696.395 70696.40 70696.395000000

Also I should add that if I do this:


$a = 8.930 + 70687.465;

instead of this:


$a = 70687.465 + 8.930;


It works.  The round() function seems to behave correctly in either case.  I 
cannot tell from this behavior if the problem is in number_format (which may not 
be calling round(), but doing its own flawed rounding) or if it something deeper 
in how PHP is storing floats/doubles.

This problem (if it is one) seems to extend to *printf* functions too:


$a = 8.930 + 70687.465;
$b = 70687.465 + 8.930;
$c = 70696.395000;
printf("A: %f, %.2f\n", $a, $a);
printf("B: %f %.2f\n", $b, $b);
printf("C: %f %.2f\n", $c , $c);'

Output:
A: 70696.395000, 70696.39
B: 70696.395000 70696.39
C: 70696.395000 70696.40

C version:


#include <stdio.h>

int main(void) {
    float a, b, c, d;
    a = 8.930 + 70687.465;
    b = 70687.465 + 8.930;
    c = 70696.395000;

    printf("A: %f %.2f\n", a, a);
    printf("B: %f %.2f\n", b, b);
    printf("C: %f %.2f\n", c, c);
}

Output:


A: 70696.398438 70696.40
B: 70696.398438 70696.40
C: 70696.398438 70696.40

Floating point values have a limited precision. Hence a value might 
not have the same string representation after any processing. That also
includes writing a floating point value in your script and directly 
printing it without any mathematical operations.

If you would like to know more about "floats" and what IEEE
754 is, read this:
http://www.floating-point-gui.de/

Thank you for your interest in PHP.

As you said - it's related to the way PHP (and almost all computers) store floating point numbers.

With all due respect, you are not reading this.

sprintf('%.2f', $float);
number_format('%.2n', $float);

should *ROUND* these numbers, the behavior is incorrect

Sorry, I meant money_format('%.2n', $float);


In all of these cases the number should be rounded to 70696.40.

If anyone else runs across this, there is a good write-up here of the problem 
(with some proposals to fix it):

https://wiki.php.net/rfc/rounding

My take-away is that when using %f in any of the *printf* functions (which I 
presume money_format uses - what others I can only imagine), the only way to get 
true correctly rounded numbers is to explicitly round() the float before using it.

Here is another example:


php -r '$a = 70687.465 + 8.930;$b = 8.930 + 70687.465;$c = 70696.395000;printf("%f 
(%.2f)  %f (%.2f) %f (%.2f)\n", $a, $a, $b, $b, $c, $c);'