A guid (global unique identifier) element is an optional subelement of item. The RSS spec states that it's a string that uniquely identifies the item. When present, an aggregator may choose to use this string to determine if an item is new.
<guid>http://some.server.com/weblogItem3207</guid>
There are no rules for the syntax of a guid. Aggregators must view them as a string. It's up to the source of the feed to establish the uniqueness of the string.
If the guid element has an attribute named "isPermaLink" with a value of true, the reader may assume that it is a permalink to the item, that is, a url that can be opened in a Web browser, that points to the full item described by the <item> element.
isPermaLink is optional; its default value is true. If its value is false, the guid may not be assumed to be a url, or a url to anything in particular.
A guid element has two methods:
$guid = $element->getGUID;
$permaLink = $element->isPermaLink();
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