ps -o rss -C httpd | tail -n +2 | (sed 's/^/x+=/'; echo x) | bc

For me it was as simple as enabling the CGI module for apache2:

a2enmod cgi
service apache2 reload

1. Edit the default apache conf file

vim /etc/apache2/apache2.conf

2. Look for the following line

Options Indexes FollowSymLinks

3. Update to

Options All FollowSymLinks MultiViews -Indexes

4. Restart apache2

/etc/init.d/apache2 restart