Padraic Brady has written up a post on how he implemented conditional fetching (GET) as a part of the Zend_Feed component of the Zend Framework.

You see, by default, Zend_Feed is stupid. It will blindly drag in whatever RSS you point it at, parse it, present an accessible API (which is largely an abstract API across PHP DOM), and then merrily sit back while you are driven demented. There is a problem in blindly fetching RSS and parsing it - RSS feeds from a huge number of online sources only change rarely. The rest of the time the feed is unchanged.

The key is in the "Last-Modified" header data of the remote file (and ETag). He shows how to use this knowledge in a simple example - pulling data with a ZFBlog_Aggregate class and dumping the contents into a database table. This code checks the return status for a 304 ("Not modified") and closes out the connection if so. Otherwise it grabs the content and updates the database with the most recent fetch times to compare to the "Last-Modified".