if i use usort, uksort etc within a class method, i cannot
specify the cmp_function as another class method. PHP seems to ignore it, and no sort is done. No error message whatsoever.
the cmp_function has to be present as a function in the calling script.
This prevents me from relegating the sort logic to a well uniquely defined method, and obliges to have a function by that exact name whenewer I want to call that class method.

In practice sorting arrays from within classes is unusable