See the right sidebar for your download options. Or build from source.

After download, unpack the archive:

$ tar xzf php-couchbase-memcached-$system-$arch.tar.gz

The resulting directory include a file memcached.so that is the PHP extension for your system.

Edit your php.ini to include this line:

extension=/path/to/memcached.so

To find where you php.ini file is, try 'phpi -i | grep memcached' or look at your <?php phpinfo() ?> output.