PHP数学奇怪的行为

Question

I have phantom bug in my code. It appears in one case from 10 attempts. When I try to debug it I found this strange behavior:

$a = floor(1385968017.8665 * 10000); // or (int) (1385968017.8665 * 10000)
// here $a equals to 13859680178664
$b = (1385968017.8665 * 10000);
// here $b equals to 13859680178665

I have such PHP version/configuration:

PHP 5.3.10-1ubuntu3.8 with Suhosin-Patch (cli) (built: Sep  4 2013 20:00:51) 
Copyright (c) 1997-2012 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2012 Zend Technologies
with Xdebug v2.2.3, Copyright (c) 2002-2013, by Derick Rethans

UPDATE: I know about floating operations precesion. Same code in C++:

#include <iostream>

int main()
{
    std::cout << std::fixed << (1385968017.8665 * 10000) << std::endl;
    std::cout << (unsigned long long) (1385968017.8665 * 10000) << std::endl;

    return 0;
}

gives me next output:

13859680178664.998047
13859680178664

And it was I expected from it. But why my PHP code shows that (1385968017.8665 * 10000) is precise equals to 13859680178665?

UPDATE 2: As user @core1024 mentioned in comments, PHP possible is rounding result of calculation (1385968017.8665 * 10000). If try php -r 'printf("%f ",1385968017.8665 * 10000);' result will be somewhere about 13859680178664.998047. Who can explain when and why PHP do this?

Answer 1

I found explanation for this problem. My php.ini precision setting is equal to 14. If I increase precision setting by one, this stragne behavior not represented:

$a = floor(1385968017.8665 * 10000); // or (int) (1385968017.8665 * 10000)
// here $a equals to 1385968017866(!)5