@aaron noticed that the php-memcached extension has various breaking changes in the release for PHP 7.
>>! <https://packages.debian.org/jessie/php5-memcached>
> php5-memcached (2.2.0-2)
>>! <https://packages.debian.org/stretch/php-memcached>
> php-memcached (3.0.1
>>!<https://pecl.php.net/package/memcached/3.0.0>
> API:
> * The method signature of get, getByKey, getMulti, and getMultiByKey changed.
> * get* and getMulti* commands **no longer take cas** or user flags parameters.
> * get* and getMulti* commands now take the Memcached::GET_EXTENDED flag to retrieve user flags and cas tokens.
> Session handler
> * The default protocol was **changed from text to binary** protocol. If your memcached setup does not support the binary protocol (e.g. if using **twemproxy**), then set memcached.sess_binary_protocol = Off. (Previously called memcached.sess_binary)