Bit not operation in PHP(or any other language probably)

Question

Why does this code return -1 ?

$a = 0; 
echo ~$a;

Isn\'t not suppose to revert the bits ?

Answer 1

Yes it should, and on the two's complement number system, a number with all bits set equals to -1 and since 0 has all bit unset, ~$a will have all its bit set.

So the code is behaving as expected.