this depend on how you installed them :
as an example you could find php-mbstring as a rpm , and if it has been installed it will be activated jsute after a restart of httpd-e-smith because the rpm create /etc/php.d/mbstring.ini
however, if you installed it using pear or pecl, you will need as pointed janet to make some templates to add the line to activivate the module in /etc/php.ini.