I guess rather than a php script, probably a shell script would be better (just because anyway it would need exec and so part of the platform compatibility would be lost or hard to keep anyway).
I guess the ideal command would be:
releaseelk.sh 1.1 beta 1
and use the three parameters (1.1, beta and 1) to build anything is necessary (e.g. download file names and repo tags).
I don't think it is so important to branch the beta out of the development branch, so no need to fetch a particular branch, just development.
So, something like:
REPO = "http://github.com/Elkarte/elkarte"
BRANCH = "development"
rm -rf ./
git clone $REPO
git checkout $BRANCH
rm -rf ./docs
rm -rf ./tests
rm -rf ./anything_needed
zip I don't remember the command
code to push to the repo both the new tag and the release (as draft?)