In Debian the config file is called apache2.conf and it's found under /etc/apache2. However, if you want to find a file called index.php it should be under the document root location. Probably /var/www or /var/www/html. The default file apache will look for is index.html, probably. You or a CMS would have to create an index.php if that's what you're using.
What is your purpose for looking for or wanting an index.php file? You gave us no details about what you're doing or why you are looking for httpd.conf ? (Sorry if I'm just feeding the troll...)