usort inside a class

Hi, i'm trying to build a class that sorts a multidimensional array. I'm using the php function "usort". I declared the comparision function as a method of my class but i'm unable to give it as argument to the function usort. this "usort($this->arr, $this->cmpi)" gaves the following error: usort() [function.usort]: Invalid comparison function i tried to do "usort($this->arr, 'cmpi')" but it does not work either. Thanks ____________________________________________________________________________________ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ