After looking trough the source code of both PECL extension and libmemcached itself I finally found my answer in the comment of the blog of the author of the memcached extension.

http://zmievski.org/2009/01/new-memcached-extension

I quote in case his blog disappeared some day:

Andrei said: @Ash, the session save path syntax is not quite the same as the other memcache extension. Try:

session.save_path="127.0.0.1:11211"

Ash Searle said: Is there any documentation for the syntax – i.e. does it handle multiple servers and optional parameters like the ea...