Use FFMPEG from PHP
Okay so this is really just a beginning.
Verify if this is a valid file
$ffmpeg = new FFMPEG('/path/to/your/video.mp4');
echo $ffmpeg->isValid(); //true;
echo $ffmpeg->isVideo(); //true;
echo $ffmpeg->isAudio(); //false;
Get metadata
$ffmpeg = new FFMPEG('/path/to/your/video.mp4');
$metadata = $ffmpeg->getMetadata();
echo $metadata['duration']; // 30 seconds
echo $metadata['width']; // 1920
echo $metadata['height']; // 1080
Convert video to flv
$ffmpeg = new FFMPEG('/path/to/your/video.mp4');
$ffmpeg->convert('/path/for/the/new/flv/video.flv');