I use a php script that generates thumbnails. On my local server everything works but when I commit it the remote server throws 'Call to undefined function imageconvolution()'. I read something about GD library and saw that it is enabled on both servers (local and remote). Any suggestions?
Imageconvolution() was introduced in 5.1.0. Have you checked to see that the version running on the server is at least that version?
Edit: Ahhh, Ubunutu aye? That old chestnut.
Ubuntu doesn't compile with PHPs GD support due to security reasons. It uses generic GD. There is a function here that replicates the functionality, but it isn't anywhere near as fast apparently.
If you have proper access to the server, you can compile proper GD support into it. Instructions: http://www.howtoforge.com/recompiling-php5-with-bundled-support-for-gd-on-ubuntu