Problem was within overflow detection: if result is less than any of two arguments, than there was overflow. But in fact there are three arguments, so with the right data overflow would not be detected: 0x00000001 + 0xFFFFFFFF+0xFFFFFFFF = 0xFFFFFFFF.

My fix changes operations to 16-bits, so there are no undetectable overflows possible.

Could you please add a test for this change?

Also, the implementation looks unnecessarily complicated. For example, this is how the same code in rhash looks like: https://github.com/rhash/RHash/blob/master/librhash/gost.c#L326

I have modified overflow detection code and added test. Feel free to move test to more appropriate place if necessary.

@nikic can you take care of this one please ?

Since all versions of PHP are affected by this bug: https://3v4l.org/0IECu, it would be very convenient if this fix would be applied to all supported PHP versions.

Sorry for the delay, now merged into 7.0+ via eac8166. Thanks!