Description:
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See first comment on:
http://www.php.net/manual/en/memcached.callbacks.read-through.php

"The expiry on read-through cache set values is set to 0, or forever.  This means if you want your key to implicitly expire, don't use the callback methods, instead check for boolean false as a return and manually set the value, at least for now."

I think this functionality is a must-have (if memcache protocol allows for it). If not then documentation should be updated to reflect it.