You certainly can do that, but you're really getting outside the scope of the PHP buildpack. The default configuration is meant to run one app, so effectively you're talking about providing a very custom HTTPD configuration.
A few pointers:
First, add a `.profile` file in the root of your project. Put this in there.
# delete the line starting with `php-fpm:`
# this makes PHP not start, which is good cause we don't need it
sed -i '/^php-fpm/d' $HOME/.procs
# unzip stie files if they are zipped
pushd site/
if [ -f "site.tar.gz" ]; then
tar zxf site.tar.gz --strip-components 1
rm site.tar.gz
fi
popdIt does two things. First, it makes it so that PHP won't actually run. You can't easily prevent it from being installed, but you can make it not run, which saves you some memory. Second it will before your app starts optionally extract a tar gzip version of your site into the `site/` directory. This can dramatically speed up the `cf push` times for your app if you have lots of small (less than 65k) files, which many static sites do. If you don't include a zip of your site, it does nothing.
Create `.bp-config/httpd/httpd.conf` and in that put your totally custom HTTPD config file. This will completely override anything from the buildpack so you need a full and valid HTTPD config file. If you don't want to completely override the buildpack, you can put just select files in `.bp-config/httpd/extra/` which override these files (the file names must match exactly). You can use a blank file to just remove behavior, like to turn off the PHP configuration. PROTIP: you can use `${HOME}` to reference the root of your project and `${PORT}` which is helpful for `Listen` and `DocumentRoot`.
Make a folder for the files you want to serve up, put that under the root of your project. If you don't care about the name, use `htdocs` (even if you're not serving up any files do this). If you need a different name, add `.bp-config/options.json` and put `{"WEBDIR": "site"}` (or replace site with the name of your public files folder. If you don't create this folder, the buildpack will move files around and it'll probably break the paths you expect in your httpd.conf file which may cause your app to fail to start.
For what it's worth, there is a Cloud Native buildpack for just HTTPD. This will make doing fun custom things with HTTPD easier in the future, when Cloud Foundry can run Cloud Native Buildpacks.
Hope that helps!