Codice calcolo pagerank

[marcio]

ho trovato questo codice,non so come funziona però
passando l'url pagina.php?url=http://www.google.it
mi restituisce

url: http://www.google.it
Checksum <2.0.114: 62914564923
Checksum >=2.0.114: 63929661791

<?php 
/* 
        Written and contributed by 
        Alex Stapleton, 
        Andy Doctorow, 
        Tarakan, 
        Bill Zeller, 
        Vijay "Cyberax" Bhatter 
        traB 
    This code is released into the public domain 
*/ 
header("Content-Type: text/plain; charset=utf-8"); 
define('GOOGLE_MAGIC', 0xE6359A60); 

//unsigned shift right 
function zeroFill($a, $b) 
{ 
    $z = hexdec(80000000); 
        if ($z & $a) 
        { 
            $a = ($a>>1); 
            $a &= (~$z); 
            $a |= 0x40000000; 
            $a = ($a>>($b-1)); 
        } 
        else 
        { 
            $a = ($a>>$b); 
        } 
        return $a; 
} 


function mix($a,$b,$c) { 
  $a -= $b; $a -= $c; $a ^= (zeroFill($c,13)); 
  $b -= $c; $b -= $a; $b ^= ($a<<8); 
  $c -= $a; $c -= $b; $c ^= (zeroFill($b,13)); 
  $a -= $b; $a -= $c; $a ^= (zeroFill($c,12)); 
  $b -= $c; $b -= $a; $b ^= ($a<<16); 
  $c -= $a; $c -= $b; $c ^= (zeroFill($b,5)); 
  $a -= $b; $a -= $c; $a ^= (zeroFill($c,3));   
  $b -= $c; $b -= $a; $b ^= ($a<<10); 
  $c -= $a; $c -= $b; $c ^= (zeroFill($b,15)); 
   
  return array($a,$b,$c); 
} 

function GoogleCH($url, $length=null, $init=GOOGLE_MAGIC) { 
    if(is_null($length)) { 
        $length = sizeof($url); 
    } 
    $a = $b = 0x9E3779B9; 
    $c = $init; 
    $k = 0; 
    $len = $length; 
    while($len >= 12) { 
        $a += ($url[$k+0] +($url[$k+1]<<8) +($url[$k+2]<<16) +($url[$k+3]<<24)); 
        $b += ($url[$k+4] +($url[$k+5]<<8) +($url[$k+6]<<16) +($url[$k+7]<<24)); 
        $c += ($url[$k+8] +($url[$k+9]<<8) +($url[$k+10]<<16)+($url[$k+11]<<24)); 
        $mix = mix($a,$b,$c); 
        $a = $mix[0]; $b = $mix[1]; $c = $mix[2]; 
        $k += 12; 
        $len -= 12; 
    } 

    $c += $length; 
    switch($len)              /* all the case statements fall through */ 
    { 
        case 11: $c+=($url[$k+10]<<24); 
        case 10: $c+=($url[$k+9]<<16); 
        case 9 : $c+=($url[$k+8]<<8); 
          /* the first byte of c is reserved for the length */ 
        case 8 : $b+=($url[$k+7]<<24); 
        case 7 : $b+=($url[$k+6]<<16); 
        case 6 : $b+=($url[$k+5]<<8); 
        case 5 : $b+=($url[$k+4]); 
        case 4 : $a+=($url[$k+3]<<24); 
        case 3 : $a+=($url[$k+2]<<16); 
        case 2 : $a+=($url[$k+1]<<8); 
        case 1 : $a+=($url[$k+0]); 
         /* case 0: nothing left to add */ 
    } 
    $mix = mix($a,$b,$c); 
    /*-------------------------------------------- report the result */ 
    return $mix[2]; 
} 

//converts a string into an array of integers containing the numeric value of the char 
function strord($string) { 
    for($i=0;$i<strlen($string);$i++) { 
        $result[$i] = ord($string{$i}); 
    } 
    return $result; 
} 


// converts an array of 32 bit integers into an array with 8 bit values. Equivalent to (BYTE *)arr32 

function c32to8bit($arr32) { 
    for($i=0;$i<count($arr32);$i++) { 
        for ($bitOrder=$i*4;$bitOrder<=$i*4+3;$bitOrder++) { 
            $arr8[$bitOrder]=$arr32[$i]&255; 
            $arr32[$i]=zeroFill($arr32[$i], 8); 
        }     
    } 
    return $arr8; 
} 


// http://www.example.com/ - Checksum: 6540747202 
$url = 'info:'.$_GET['url']; 
print("url:\t{$_GET['url']}\n"); 
$ch = GoogleCH(strord($url)); 
printf("Checksum <2.0.114:\t6%u\n",$ch); 

$ch=sprintf("%u", $ch); 
// new since Toolbar 2.0.114 

$ch = ((($ch/7) << 2) | (((int)fmod($ch,13))&7)); 

$prbuf = array(); 
$prbuf[0] = $ch; 
for($i = 1; $i < 20; $i++) { 
      $prbuf[$i] = $prbuf[$i-1]-9; 
} 
$ch = GoogleCH(c32to8bit($prbuf), 80); 
// 

printf("Checksum >=2.0.114:\t6%u\n",$ch); 
?>

qualcuno sa dare una spiegazione?

mi sa che non è uno script per calcolare il pr
è per ottenere il checksum di un sito con il quale mi sembra si controlli il pr, in pratica è incompleto

ma cos'è il checknum e il pr?