php modulo return wrong result

Question

I got problem with this code:

if (!empty($_GET[ "lic" ])) $lic = $_GET[ "lic" ]; else $e = true;
echo ($lic % 11);

When I post 8911076856 it echoes 1, but it should be 0.

Might that be because 8911076856 is too high for an int, and overflowing? — andrewsi
– andrewsi, Commented Aug 22, 2012 at 14:03
I think that's not the problem as var_dump(8911076856 % 11); returns int(0). Call var_dump($lic) before echoing the modulo. — Florent
– Florent, Commented Aug 22, 2012 at 14:06
@Florent - that is the problem. It's a 32bit system and he's trying to handle value too large to fit 4 byte integer. You probably tested on 64 bit system. — N.B.
– N.B., Commented Aug 22, 2012 at 14:08
@Florent: In that operation, the number 8911076856 is parsed as a float, but cast internally to an integer for the modulus. Try var_dump((int)8911076856); to see what it's actually using in the operation. — FtDRbwLXw6
– FtDRbwLXw6, Commented Aug 22, 2012 at 14:09
probably its 32b system, i dont know it exactly (Apache/2.2.15 (Debian) Server), but if it doesnt work, than is easy to find out — Ondro Tadanai
– Ondro Tadanai, Commented Aug 22, 2012 at 16:43

oopbase · Accepted Answer · 2012-08-22 14:05:20Z

9

The value "8911076856" is probably above the maximum integer value of your system.

echo ((int)8911076856);

My result is 321142264 on my 32 Bit system.

answered Aug 22, 2012 at 14:05

oopbase

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2 Comments

Touki Over a year ago

That's right, I got the right result (0) on a 64bits system

Sjoerd Over a year ago

In this case, PHP automatically converts the number to a float. Apparently, the modulo operator does not work correctly on floats.

Sjoerd · Accepted Answer · 2012-08-22 14:06:35Z

7

Use fmod:

echo fmod(8911076856, 11);

answered Aug 22, 2012 at 14:06

Sjoerd

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2 Comments

FtDRbwLXw6 Over a year ago

+1. Out of curiosity, is the benefit of fmod() over (float)$x % (float)$y just the precision of the result?

Sjoerd Over a year ago

No, (float)$x % (float)$y does not give the correct result for values of $x above PHP_INT_MAX. E.g. (float)8911076856 % (float)10 gives 4 instead of 6. Apparently the modulo operator casts both arguments back to int.

FtDRbwLXw6 · Accepted Answer · 2012-08-22 14:50:44Z

2

This is most likely being caused because the number you're posting is higher than PHP_INT_MAX, which is 9223372036854775807 on most 64-bit systems AFAIK. If you're using a 32-bit system (which I expect you are), it's probably 2147483647.

edited Aug 22, 2012 at 14:50

answered Aug 22, 2012 at 14:06

FtDRbwLXw6

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2 Comments

Sjoerd Over a year ago

On 32-bit machines, it is 2147483647.

FtDRbwLXw6 Over a year ago

@Sjoerd: You're right, my mistake. I corrected my answer. Forgot I was on my laptop. :-P

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