I seen where a lot of server management companies are charging big bucks for this, so this might save you some time and money.
nano -w /etc/yum.repos.d/dag.repo
name=Dag RPM Repository for Red Hat Enterprise Linux
Then yum update and search for ffmpeg and install all ffmpeg packages including devel.
Seems there is some lib problems along the way so in /etc/ld.so.conf
To install the php extension follow the simple directions on http://ffmpeg-php.sourceforge.net/
Then you should be all set!