First, remove ffmpeg-php and any ffmpeg rpm packages you installed, including the devel packages.
Next, you'll want to try and manually remove the files you installed. The directories for the includes are "libavcodec", "libavdevice", "libavformat", "libavutil", and "libpostproc". You might also have "libswscale", although it depends on if it was a separate package or not. You will also want to hunt down the so files of those names as well.
The ffmpeg binaries are "ffmpeg", "ffplay", and "ffserver". Man pages use those names as well.
That should get you started on cleaning things out to start fresh.