Unsigned right shift function not working for negative input

Question

I\'m looking for a way to use the >>> function from JavaScript in the 64-bit version of PHP 5.5.14. I found this function in my googling:

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Answer 1

The constant 0x80000000 (it's written as a call to hexdec and stored in the $z variable in this example) represents the lowest signed two's complement negative integer (100000.... in binary). The expression ~$z should give the bitwise NOT of this, namely, the highest signed positive integer (which ends up being 2147483647).

The original number (positive 0x80000000, that is, 2147483648) cannot be stored as a signed 32-bit integer, so normally it would be stored as a float of some kind. Unfortunately, PHP 5.5 thinks that ~(2147483648) is equal to -2147483649, which would be correct if we were dealing with e.g. 64-bit integers.

And indeed, echoing out PHP_INT_SIZE in runnable indicates that integers are 8 bytes, which is 64 bits. Thus, the arithmetic isn't working out right in PHP 5.5.

To remedy this, just replace the ~$z with a static constant, as follows:

function uRShift($a, $b) 
{ 
    if ($a < 0) 
    { 
        $a = ($a >> 1); 
        $a &= 2147483647; 
        $a |= 0x40000000; 
        $a = ($a >> ($b - 1)); 
    } else { 
        $a = ($a >> $b); 
    } 
    return $a; 
}

This function still has some weaknesses; for example, shifting by 0 doesn't work properly.