This page – https://community.c9.io/t/setting-up-phpmyadmin/1723 – says it all, but here are the keys points again:
Using PHPMyAdmin with Cloud9
First, create a PHP workspace so you have PHP, MySQL, and Apache installed right away. You can then make sure you have MySQL installed by running:
Then install phpMyAdmin:
After the installation is complete you’ll just want to make sure mysql is running once more:
After installing phpMyAdmin, you were given a link to access PHPMyAdmin which will follow the pattern of: https://[workspacename]-[username].c9users.io/phpmyadmin. From that link, you can log in with your Cloud9 username and a blank password.