PHP Code:
$array = array('a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z');
shuffle($array);
$string = $array[0].$array[1];
edit: nevermind, seems you wanted a pattern like that not random. didn't read properly