Ref T9640. Recent versions of APCu only define apcu_* functions.
- Maniphest Tasks
- T9640: Make Phabricator compatible with PHP7
- rPHUa2dbae060a5e: In PhutilAPCKeyValueCache, use APCu functions on APCu 5+
See D17198, which has a screenshot of the cache full of data after this change.