Kevin Waterson has posted a new tutorial to the PHPro.org website today stepping you through the automation of handing reciprocal links.
This tutorial looks at the process of reciprocal links. That is, links generated from websites that have links to your own page. These links back to a page can be detected from the HTTP REFERER which, in PHP, can be detected via the super global variable $_SERVER['HTTP_REFERER']. Care should be taken when using this variable as it is set from userland and, as such, should not be trusted.
His method stores the linking information (gathered from the referrer) and calls an addLinks method that checks to ensure the page format is valid and that it's a real site before putting it into the table. Complete code for this, a fetch method and a simple database layer to put it on top of are included.