What is the necessary logic to create a Sudoku in PHP?

I am interested in creating a website that would allow users to make and print their own Sudokus(http://en.wikipedia.org/wiki/Sudoku). I can't think of the necessary logic to do this using PHP.

What is the necessary logic to create a Sudoku in PHP?
Here is a description of a site which creates sudoku puzzles, and the algorithm they use (but not code or finer details) on how to create a puzzle.
Reply:See alternatively http://ksudoku.sourceforge.net/2.htm