The best way to install MongoDB on yum-enabled Linux distributions (CentOS or Fedora) is through yum package manager. MongoDB.org provides repositories (depending on your OS and CPU architecture) that you should add to your list of yum repos:
http://www.mongodb.org/display/DOCS/CentOS+and+Fedora+Packages

Once you have the repository properly installed under /etc/yum.repos.d you can install the latest stable mongoDB release with:\

sudo yum install mongo-stable-server

which will actually install both the server and the client (since client is set as a dependency).

Once installed MongoDB can be started with:\

sudo /etc/init.d/mongod start

Further, if you need to Mongo-enable your PHP just run:\

sudo pecl install mongo

and follow instructions.