PHP - right shift operator

The Bitwise right shift operator (>>) takes the two numbers and right shift the bits of first operand by number of place specified by second operand. For example: for right shifting the bits of x by y places, the expression (x>>y) can be used. It is equivalent to dividing x by 2^y.

The example below describes how right shift operator works:

1000 >> 2 returns 250

                      (In Binary)
   1000         ->    1111101000  
   >> 2                     |  right shift the bits
   -----                    V  by 2 places
    250         <-      11111010 
                      (In Binary)

The code of using right shift operator (>>) is given below:

<?php
$x = 1000;
$z;

//right shift operation
$z = $x >> 2;

//Displaying the result
echo "z = $z";
?>

The output of the above code will be:

z = 250

Example: Find largest power of 2 less than or equal to given number

Consider an integer 1000. In the bit-wise format, it can be written as 1111101000. However, all bits are not written here. A complete representation will be 32 bit representation as given below:

00000000000000000000001111101000

Performing N = N | (N>>i) operation, where i = 1, 2, 4, 8, 16 will change all right side bit to 1. When applied on 1000, the result in 32 bit representation is given below:

00000000000000000000001111111111

Adding one to this result and then right shifting the result by one place will give largest power of 2 less than or equal to 1000.

00000000000000000000001000000000

The below code will calculate the largest power of 2 less than or equal to given number.

<?php
function MaxPowerOfTwo($N) {
  //changing all right side bits to 1.
  $N = $N | ($N>>1);
  $N = $N | ($N>>2);
  $N = $N | ($N>>4);
  $N = $N | ($N>>8);
  $N = $N | ($N>>16);
  
  //adding 1 to $N makes smallest power
  //of 2 greater than given number
  $N = $N + 1;

  //right shift by one position makes
  //largest power of 2 less than or 
  //equal to given number
  $N = $N >> 1; 
  return $N;
}

echo "MaxPowerOfTwo(100) = ".
    MaxPowerOfTwo(100)."\n";
echo "MaxPowerOfTwo(500) = ".
    MaxPowerOfTwo(500)."\n";
echo "MaxPowerOfTwo(1000) = ".
    MaxPowerOfTwo(1000)."\n";      
?>

The above code will give the following output:

MaxPowerOfTwo(100) = 64
MaxPowerOfTwo(500) = 256
MaxPowerOfTwo(1000) = 512

❮ PHP - Operators