web: vendor/bin/heroku-php-apache2 your/public/root
web: vendor/bin/heroku-php-nginx your/public/root
Simply omitt your/public/root to point to the root of your repository.
You can also try HHVM runtime. Just change php with hhvm and that's all.
If you specify additional options to the script (as described in the sections that follow), you must ensure that the document root argument is at the end of the command, after all options.
For example:
web: vendor/bin/heroku-php-nginx -C rewrite.conf www/