Since the module is not included with the default PHP installation, you have to download it from the official repository:

cd
mkdir mongo-php-driver
cd mongo-php-driver
curl https://codeload.github.com/mongodb/mongo-php-driver/zip/master > …

The first step is to configure the repositories. Create the following file /etc/yum.repos.d/10gen.repo with the following contents:

For 32-bit systems:

[10gen]
name=10gen Repository
baseurl=http://downloads-distro.mongodb.org/repo/redhat/os/i686 …

Installing PHP/MongoDB extension is a two steps task on OSX:

• Install the autoconf tool required for compiling the extension
• Install the Mongo extension

You have to install autoconf in order to avoid the following error:

Cannot find autoconf. Please …