Ok it seems there is in fact a php5-memcached on dotdeb and it's that which requires libmemcached11.
That's in the main wheezy repository there, which you can add with the following sources.list lines
deb http://packages.dotdeb.org wheezy all
deb-src http://packages.dotdeb.org wheezy all
From:
https://www.reddit.com/r/debian/comments/1rmz2e/ho...