This git repository helps you get up and running quickly w/ a PHP sample application that uses MongoDB on OpenShift
Create an account at https://www.openshift.com
Create a php application with mongodb
rhc app create phpmongotweet php-5.3 mongodb-2
Add this upstream phpmongotweet repo
cd phpmongotweet
git remote add upstream -m master git://github.com/openshift-quickstart/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