## 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

`<?php
$m = new Memcached();
$m->addServer('localhost', 11211);
$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;
return true;
}
?>`