I don't remember having any way of doing that in pure-php :-(
Generally, the choosen solution is to call ffmepg in command-line, with stuff like shell_exec
But, if you can't install software on your server, this will probably not be possible (I doubt your hosting provider bundles ffmepg on their servers)
There is even an extension to use ffmpeg from PHP without having to call it via command line : ffmpeg-php
But, as it's a PHP extension (and not just a bunch of PHP scripts), you'll have to install it on your server -- and we're probably back to the same problem :-(
Searching a bit, I found AllBrand.nu Automagic Thumbnailer ; they say (quoting) :
Features:
* ...
* Supports jpeg, gif, png, wbmp and video files
* ...
Requirements:
* PHP capable web server with GD support
* ...
But it doesn't seem to be really known... So not sure it works well, nor that it's a good solution...
Anyaway... Good luck !
And if you find some working / nice solution, don't forget to let us know !