An explanation and example from a blackhat blog:
"So you want to get into link exchanging, but no one wants to trade with you because your page rank sucks.
Guess you need to spoof a reasonable Google PR like a 5, 6 or 7.
Well the good folks at SEOXXXXXXXX (among others) demonstrate how you can spoof or fake a PR 10 or any Page rank you want.
Here?s how you Spoof a Google Page Rank:
1. Buy a domain.
2. 301 Redirect the domain either in you .htaccess, PHP header (anything server side) to a page with the desired page rank. It?s better to redirect to something relevant. For example, if you are doing a mortgage site, redirect to a mortgage site.
for php the code is:
<?php
header(?HTTP/1.1 301 Moved Permanently?);
header(?location:
http://www.XXXXXXXXXXXXXXXXXXXX.com?);
exit;
?>
3. Link to the page with the redirect from another of your sites that gets regularly spidered by Google. Since you use the Alexa Toolbar, you should have several sites that are spidered daily by google.
4. Wait for google update. Google Updates happen about every three months or now perhhaps even more often.
5. Now you use cloaking software that says:
<?php
if(ip is in list) (list of google IPs){
header(?HTTP/1.1 301 Moved Permanently?);
header(?location:
http://www.XXXXXXXXXXXXX.com?);
exit;}
else
{
header(?Content-Type: text/html; charset=ISO-8859-1′);
//Show your site
};
?>
The main drawback is that your site will not actually be indexed by Google as long as you are redirecting the Googlebots to your competitors site, so it?s really only good for link selling / trading.
BTW, don?t claim to have a certain PR when you are selling ? that?s fraud.
This further illustrates how useless Page Rank has become in measuring a sites importance.
Buyer Beware."
"Detect Fake Pagerank
To detect Fake Pagerank, simply type in the webpage, such as
www.SiteWithFakePR.com into Google?s search and look at the cache. The cache will not be that of the site with Fake PR, it will be the site with the actual PR."