-memcached_delete() is used to delete a particular key. An expiration value
-can be applied so that the key is deleted after that many seconds.
+memcached_delete() is used to delete a particular key.
+memcached_delete_by_key() works the same, but it takes a master key to
+find the given value.
+
+Expiration works by placing the item into a delete queue, which means that
+it won't possible to retrieve it by the "get" command, but "add" and
+"replace" command with this key will also fail (the "set" command will
+succeed, however). After the time passes, the item is finally deleted from server memory.
+
+Please note the the Danga memcached server removed support for expiration in
+the 1.4 version.