Best way to invalidate a number of memcache keys using standard php libraries?

Question

I have a database with files which can be searched, browsed and have multiple copies on multiple servers.

I cache searches, browse pages and server locations (urls). Say

Answer 1

I have no experience with memcached, but I understand that IO are cheap there.

I'd go with your tag implementation, make sure the tag list is used frequently and hope that the internal mmcd' logic would "think" that it's something too busy to be dropped :)