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 ยป 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.
If you install it by: "pecl install memcached", don't forget to install libmemcached-devel too. You can find it in remi repo.