How do I make an RSS subscribers counter?

I’m trying to make an RSS subscribers counter using PHP and don’t know where to start. I’m not asking someone to write me the code, but do you know something good to read? or can you g开发者_如何学Pythonive me the general steps to do that?

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A quick count of how many times the RSS feed has been requested would be a good start, I suppose. Also, if several people subscribe using Google Reader, or Gmail (which puts RSS feeds at the top of the inbox), Google will, of course, query your feed only once. But, in the User-Agent string, it will tell you how many subscribers you currently have through that service. I imagine other feed aggrigators do something similar.

Also, Internet Explorer uses different User-Agent strings. If the user opens a feed in his browser, you get the normal IE User-Agent string, but if the browser is checking the feed automatically because the user is subscribed, you get a different string.