How can I figure out which PHP extensions aren't being used?

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http://serverfault.com – I manage a server (running Ubuntu) which hosts our client's sites with a few dozen different PHP-based websites, mostly small sites but also some installations of CMSes and forums. I used the get_loaded_extensions() method to see what extensions I have loaded. To help streamline the server (remove unnecessary extensions to make upgrading easier and marginally improve speed), I'd like to remove extensions that aren't being used by any of the sites. I currently have 54 different extensions loaded. I can easily eliminate some of these from the list which I know are used, but others I am less s (HowTos)