Why does “$temp = 0; echo ~$temp;” print -1?

Question

Can anybody tell the internal procedure of the below code

<? $temp = 0; echo ~$temp; ?> 
   //instead of 1 it displays -1

Answer 1

echo ~$temp;
     ^bitwise not operator

Assuming 32-bit, Bitwise inverse of 0000 is FFFF (All 1's) which is -1, in signed int's case.

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Another way to look at it: What ~ did is to give you the (One's complement)

In order to get the negative of a number, you take the 2's complement, which is just the 1's complement + 1

So,

(1's complement of 0) + 1 = 0 //negative 0 is 0
hence, 1's complement of 0 = -1

Answer 2

Bitwise-not (~):

This inverts each bit of its operand. If the operand is a floating point value, it is truncated to an integer prior to the calculation. If the operand is between 0 and 4294967295 (0xffffffff), it will be treated as an unsigned 32-bit value. Otherwise, it is treated as a signed 64-bit value

Its because you're actually dealing with a full 32-bit unsigned integer with NOT. What that means is you're not simply inverting 0001 but inverting 00000000000000000000000000000001

which becomes 11111111111111111111111111111110

essentially this is the number + 1 and negated. so 1 becomes -(num+1) which is -1 or 1111111111111111111111111111110 in binary (unsigned)

for example:- $temp=1; echo~$temp; print -2 //-(n++)