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

The integers are stored in 2's compliment form .

this form can be outline ad follows :

1) If the number to be stored is an positive value then its binary value is stored

eg $val = 5 ;

here $val contains ordiary binary equivlent of 5 = 0101 //number of bits depends on specifics

2) if you are storing negative of an number say -5 , then the twos compliment s stored

$val = -5;

here first 2's compliment of 5 is found out ie, simply 1's compliment of 5 + 1

~ 0101 = 1010

tnen add 1

  1010 + 
     1
  -----
  1011

and this 1011 gets stored in $val .

in same way , $val=0 ; 00 is stored

~$val => 11 which is the equalient of -2 in 2's compliment form

and finally , if you observe carefully , you may ask ,

so how could I represent 11 ? for its binary values is 1011 that clashes with -5's value in 2's comp ?

the answer lies in number of bits used to represent number.

in 2's complimwent form if there are n bits then you can represent only values from

-2^(n-1) to 2^(n-1) -1 ;