If you install it by: "pecl install memcached", don't forget to install libmemcached-devel too. You can find it in remi repo.
This extension requires » libmemcached client library (version 1.0.0 or greater). For SASL authentication support, libmemcached must be built with SASL enabled.
Since version 0.2.0, this extension requires PHP version 5.2.0 or higher.