usort comparison function

The usort function allows an array to be sorted on the basis of a user-supplied function that compares two elements, like usort($arr,cmp). A reader-annotation in the manual points out that the user function cmp() cannot be a member of a class and this seems to be consistent with my experiments, but it is somewhat ugly. Does anyone know if there is a workaround for this? Is it possible to use a comparison function that is anything other than a global function? Specifically, I want to use this->cmp().