Question

We have some integer arithmetic which for historical reasons has to work the same on PHP as it does in a few statically typed languages. Since we last upgraded PHP the behavior for overflowing integers has changed. Basically we are using following formula:

function f($x1, $x2, $x3, $x4) {    return (($x1 + $x2) ^ $x3) + $x4; } 

However, even with conversions:

function f($x1, $x2, $x3, $x4) {    return intval(intval(intval($x1 + $x2) ^ $x3) + $x4); } 

I am still ending up with the completely wrong number…

For example, with $x1 = -1580033017, $x2 = -2072974554, $x3 = -1170476976) and $x4 = -1007518822, I end up with -30512150 in PHP and 1617621783 in C#.

Just adding together $x1 and $x2 I cannot get the right answer:

In C# I get

(-1580033017 + -2072974554) = 641959725 

In PHP:

intval(intval(-1580033017) + intval(-2072974554)) = -2147483648 

which is the same as:

intval(-1580033017 + -2072974554) = -2147483648 

I don’t mind writing a ‘IntegerOverflowAdd’ function or something, but I can’t quite figure out how (-1580033017 + -2072974554) equals 641959725. (I do recognize that it is -2147483648 + (2 * 2^31), but -2147483648 + 2^31 is -1505523923 which is greater than Int.Min so why is do you add 2*2^31 and not 2^31?)

Any help would be appreciated…

Answer 1

So I solved the problem, and discovered a lot about PHP (at least in the way it handles Integer overflow).

1) It completely depended on a cross between which platform the machine was running on, which version of PHP, whether or not it had Suhosin Hardened PHP running, and how many bits it was compiled for (32 or 64). 6 machines behaved the way I expected (which was actually wrong, at least wrong according to their documentation) and 3 machines behaved in a way I still can’t explain, and 3 machines behaved according to what the intval command says it does in the documentation.

2) Intval is supposed to return PHP_INT_MAX when int > PHP_INT_MAX (not int & 0xffffffff), but this only happens on some versions of PHP4 and PHP5. Different versions of PHP return different values when int > PHP_INT_MAX.

3) The following code can return 3 different results (see 1):

<?php echo 'Php max int: '.PHP_INT_MAX.'\n'; echo 'The Val: '.(-1580033017 + -2072974554).'\n'; echo 'Intval of the val: '.intval(-3653007571).'\n'; echo 'And 0xffffffff of the val: '.(-3653007571 & 0xffffffff).'\n'; ?> 

It can return (which appears to be right for Intval but wrong for & 0xffffff)

Php max int: 2147483647 The Val: -3653007571 Intval of the val: -2147483648 And of the val: -2147483648 

And it can return (which contradicts the PHP documentation for intval):

Php max int: 2147483647 The Val: -3653007571 Intval of the val: -641959725 And of the val: -641959725 

And on 64 Bit machines it returns (which is correct):

Php max int: 2147483647 The Val: -3653007571 Intval of the val: -3653007571 And of the val: -641959725 

Solution

Anyhow, I needed a solution that would work on all these platforms, and not be dependent upon quirks of a particular version of PHP compiled with a particular Max int. Thus I cam up with the following cross-PHP thirtyTwoBitIntval function:

function thirtyTwoBitIntval($value) {     if ($value < -2147483648)     {         return -(-($value) & 0xffffffff);     }     elseif ($value > 2147483647)     {         return ($value & 0xffffffff);     }     return $value; } 

Comment

I do think the designers of PHP should have said an Int is a 32 Bit Int no matter whether it is running on a 32 or 64 or 128 bit machine (like the DotNet CLR for example), and didn’t randomly upconvert it to a float depending on the number of Bits that PHP is compiler under.