I think this might have to do with our use of the
apc_* functions which were available with older version of the APC extension. APCU took a subset of that and renamed the functions to
apcu_*. We should probably update our code to use the apcu_* calls rather than apc_*. If you don’t want to wait until then, there’s a small backward compatibility module you can install that will provide the older function names. If you’re able to install that, Full-Text RSS should report that all is well (since it uses the older function names).
See PECL :: Package :: apcu_bc
Having said that, other than slight increase in performance, you shouldn’t notice much difference if you don’t use APC at all, so in terms of results, you can safely ignore it.