I see a similar issue from early last year, but I don’t know which environments that covered.
Switching from Xenial to Bionic, I get the unable to load memcached error. I didn’t see any notes about changes in Bionic or the php language setup, but I tried installing the memcached extension manually through pecl:
pecl install memcached
However, this hangs on a user prompt about the libmemcached directory:
libmemcached directory [no] :
Is the memcached module support be automatically setup and loadable for bionic?