1. yum install php-devel
2. create the directory for mongo's source
3. in the directory above do:
git clone http://github.com/mongodb/mongo-php-driver.git
4. sudo phpize
5. sudo ./configure
6. sudo make
7. sudo make install
now we should have: /usr/lib/php/modules/mongo.so
8. in php.ini, add extension=mongo.so