Enables Pingback services

The Module consist of two components: The pingback server and the pingback client.

The client
The client starts his work each time when an active news entry is saved. It crawls the news text for external links and sends a request to each of them in order to find out, whether these are pingback enabled websites. If so, it sends a pingback and saves the answer to the system log.

The server
The server's job is to receive pingbacks from other sites. If he does so, he saves the incoming pingbacks as comments to the datbase and notifies the administrator by e-mail.

Installation
Install the module as usual, then add the following entry under "Additional head-tags" on each sitelayout containing a newsreader:
[link rel="pingback" href="http://www.YourDomainGoesHere.com/system/modules/z_pingback/Pingback_Server.php" /] (subtitute the square brackets with angle brackets.)

This Website may be used to test the module:
http://www.selected-stuff.de/index.php/stuff_reader/items/Pingback_Test.html

Note
Saving a news entry may last up to 10 sec. longer after installing the module.

Further links concerning pingbacks
http://en.wikipedia.org/wiki/Pingback
http://www.hixie.ch/specs/pingback/pingback
http://www.xmlrpc.com/spec
http://webmastercoffee.com/blog/pingback-bei-backlink-rueckruf