Its weired that even after I 1) changed the file name from "php.conf" to "php.bak" (so that the php module wouldn't be loaded); and 2) enabled
"HTTPD=/usr/sbin/httpd.worker" in "/etc/sysconfig/httpd".
After I restarted the httpd service successfully (no conflict with php), I still found that the "prefork" MPM was running in the server.
[root@poplar sysconfig]# httpd -l
Compiled in modules: