Is there any Imagemagik like php-plugin that can work with video files (flv in particular) ? Id want to do things like save, size, and, hopefully, merge videos.
If php isnt the most practical approach, what language would be? It needs to be web compatible.
Have you looked at phpvideotoolkit? I believe this will be what you are looking for. It isn’t a plugin but is a PHP Wrapper for video editing tools like FFmpeg, FLVTools2 and Mencoder.
EDIT:
Per the comments above these tools are mainly C / C++ which is much faster than PHP. The phpvideotoolkit wrapper will basically exec them from your PHP site which will be faster than manually using PHP to open a file and parse all the binary data.