This class will NOT EXIST if the APC extension is installed but disabled in your php.ini.
This is an important difference to functions like apc_store(), which will still exist and be callable even when APC is disabled. In many installations, the default settings are
apc.enabled = on
apc.enable_cli = off
which means that APCIterator exists for HTTP requests, but not with the "cli" SAPI. The solution is to either set "apc.enable_cli" to "on" (with the resultant startup penalty for CLI scripts), or to check the relevant ini settings before your script tries to access APCIterator.