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

That zero is actually being represented by 32 zero bits, as PHP integer types are 32-bit signed integers, and not single bits:

0000 0000 0000 0000 0000 0000 0000 0000

So bitwise NOT flips all of them, resulting in the two's complement of -1 (with the leftmost one representing the sign):

1111 1111 1111 1111 1111 1111 1111 1111