# -- unconditionally sets a given key with a given value 
#(update_foo() should use this)
$memd->set();

#-- adds to the cache, only if it doesn't already exist 
#(get_foo() should use this)
$memd->add(); 

#-- sets in the cache only if the key already exists 
#(not as useful, only for completeness)
$memd->replace();