Or just set the value within the callback with your own custom expiration time and return false. I think it's cleaner.
Read-through cache callbacks
Read-through cache callbacks are invoked when an item cannot be retrieved from the server. The callback is passed the Memcached object, the requested key, and the by-reference value variable. The callback is responsible for setting the value and returning true or false. If the callback returns true, Memcached will store the populated value on the server and return it to the original calling function. Only Memcached::get() and Memcached::getByKey() support these callbacks, because the memcache protocol does not provide information on which keys were not found in the multi-key request.
Example #1 Read-through callback example
$m = new Memcached();
$profile_info = $m->get('user:'.$user_id, 'user_info_cb');
function user_info_cb($memc, $key, &$value)
$user_id = substr($key, 5);
/* lookup profile info in the DB */
/* ... */
$value = $profile_info;
This isn't specified anywhere, so I had a gander at the source...
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.
the expiration time set for you if you return true from the callback will be 0 (forever) - so if you want a different expiration time you can do the SET operation inside the callback with your custom expiration time. just make sure you return FALSE from the callback to prevent the client from automatically setting the value again.