Via Gregor's del.icio.us feed, I found the xslcache extension for PHP published by the geeks at The New York Times, based on the original xslt extension. It caches the parsed XSLT stylesheets into your apache child memory (shared memory is on the todo list) and reuses it at the next request. This is something I wanted to do since a long time, but never did find the time for it. As ā depending on the size of your stylesheets ā the importing of XSLT stylesheets may take a considerable amount of time, this extension may really improve your website's performance. Many thanks to the NYT for publishing this. If I find the time some day, I will look more closely into it and check, if this could be integrated somehow into the mainstream xslt extension. Or if anyone else feels like it: Patches are welcome :)
There's also another interesting looking PHP extension worth checking out there: DBSLayer.