I did the following (to use with Codeigniter):
Downloaded from https://github.com/mongodb/mongo-php-driver/downloads
*Careful to get the thread-safe version.
Then add this line in your php.ini file:
extension=php_mongo-1.2.11-5.3-vc9-x86_64.dll (or whatever version did you download)
Then restart your server and check the error log to see if there are any errors/conflicts.
PS: I use WAMP!