Se você tiver GNU stdbuf , você pode usar:
$ stdbuf -o0 -e0 php -S 0.0.0.0:12345 | tee -a access.log
PHP 5.4.34-0+deb7u1 Development Server started at Mon Nov 24 14:38:33 2014
Listening on http://0.0.0.0:12345
Document root is /home/cuonglm
Press Ctrl-C to quit.