We're seeing an issue where Moodle is caching boolean data, which works perfectly well on file, memcached and mongodb caches, but not on the memcache one.

The line of code is https://github.com/moodle/moodle/blob/master/lib/weblib.php#L1729, which is setting a boolean true, and checking it's identically equal to boolean true when it comes back, but the memcache store is returning it as an integer 1. The manifestation of the issue is that strings unchanged by HTML purifier are all returned as "1".

It seems that all the other core cache stores are serialising the PHP data one way or another, but memcache isn't.