Distributing a small PHP application

Question

I wrote a small PHP application that I'd like to distribute. I'm looking for best practices so that it can be installed on most webhosts with minimal hassle.

Briefly: It's simple tool that lets people download files once they login with a password.

So my questions are:

1) How should I handle configuration values? I'm not using a database, so a configuration file seems appropriate. I know that other php apps (e.g. Wordpress) use defines, but they are global and there is potential that the names will conflict. (Global variables also have the same problem, obviously.) I looked at the "ini" file mechanism built into PHP. It only allows comments at the top - so you can't annotate each setting easily - and you can't validate syntax with "php -f". Other options?

2) How to handle templating? The application needs to pump out a form. Possibly with an error message. (e.g. "Sorry, wrong password.") I've have a class variable with the HTML form, but also allow an external template file to be used instead (specified in the config). I do some trivial search and replace - e.g. %SCRIPT% to the name of the script, %STATUS% to hold the error message. This feels a bit like reinventing the wheel, but including a templating system like Smarty is overkill. (Plus they may already have a templating system.) Other options?

3) i18n - There are only 3 message strings, and gettext doesn't seem to be universally installed. Is it such a bad idea just to make these three strings parameters in the config file?

4) How to best integrate with other frameworks? My app is a single class. So, I thought I could just include a php script that showed how the class was called. It would be a starting point for people who had to integrate it into another framework, but also be fine as-is for those not interested in customizing. Reasonable?

5) GET/POST parameters - Is it bad form for a class to be looking at $_GET and $_POST? Should all values be passed into my class during construction?

Thanks.