If you use Gentoo/CentOS with Apache2 and PHP, and PHP is not interpreted, you should do the followings:
Open Apache2's configurations:
Change the line:
APACHE2_OPTS="-D DEFAULT_VHOST -D INFO -D SSL -D SSL_DEFAULT_VHOST -D LANGUAGE -D PHP5 -D USERDIR"
After changing the line, don't forget to restart Apache:
Now it should work!