It is better set PHP_LSAPI_MAX_REQUESTS=500 or 1000. You don't want a child php process just serves one request and exits. Max Idle time by default (not set) is -1 (server never kill the parent PHP process). Another possibility of opcode cache reset is server restart (by itself). You can verify the server uptime in realtime stats and see if it does restart often (uptime is short). if so, you need to check error.log around the time it restarts for clue.