Codice calcolo pagerank

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

Codice PHP:

---

<?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?