For memcached server we are running on version 1.4.5, and the application is built with PHP 5.3.6 and memcache (http://pecl.php.net/package/memcache) version 3.0.5
Currently we have 2 memcached pools, each pool currently have 3 memcached servers in it. Meanwhile because of the need of doing load balancing for our web applications, now we have two application servers connecting to the same memcached pools. Because the first application server is installed long time ago, the memcache version is 3.0.5, for the newer server it is installed with memcache version 3.0.6.
After that something weird happened, that sometimes the data cannot be retrieved and we get errors like:
[Mon Oct 03 19:55:56 2011] [error] [client 192.168.0.22] PHP Notice: unserialize(): Error at offset 0 of 9648 bytes in /public_html/test.php
So it seems when we matched the memcache library to same version (both pools running with 3.0.6 or 3.0.5) the problems is solved. But not sure if anyone have similar issues? that which version you will recommend?
Lastly, is it better to use version 2.2.6 which is the "Stable" version?
pecl/memcache 3.0.x alpha was abandoned for a long time, then eventually someone fixed a few things.
the 2.2.x stable had a lot fewer bugs.. you are technically running an alpha release.
If you want to spend some time on it, the pecl/memcached client tends to be superior, and is under some active maintainership. The "stable" release up there is a bit old, but new work is happening.
Either way, I would keep all my software on the same version if at all possible, and you should upgrade the daemon at some point.