This git repository helps you get up and running quickly w/ a PHP sample application that uses MongoDB on OpenShift
Create an account at http://openshift.redhat.com/
Create a php-5.3 application
rhc app create -a phpmongotweet -t php-5.3
Add MongoDB support to your application
rhc app cartridge add -a phpmongotweet -c mongodb-2.0
Add this upstream phpmongotweet repo
cd phpmongotweet
git remote add upstream -m master git://github.com/openshift/phpmongotweet-example.git
git pull -s recursive -X theirs upstream master
Then push the repo upstream
git push
That's it, you can now checkout your application at: http://phpmongotweet-$yourlogin.rhcloud.com