How to install and run phpize

I have been meaning to install ffmpeg as an extension to my PHP setup. So before I can install it, I need to phpize it. I installed php5-dev by sudo apt-get install php5-dev. But now when I run phpize I get the following error :

Fastest way to extract a specific frame from a video (PHP/ffmpeg/anything)

I have a web page, which (among other things) needs to extract a specific frame from a user-uploaded video. The user seeks to a particular part of a .mp4 in the player, then clicks a button, and an ajax call gets fired off to a php script which takes the .mp4, and the exact time from the video, and uses that to extract a “thumbnail” frame.