Description:
------------
Currently, I'm using this function to determine whether a function is called statically:

function IsCalledStatically() {
    $debugBt = debug_backtrace();

    return $debugBt[1]['type'] == '::';
}

Having to rely on the debugger is not very efficient here. Other methods checking whether $this is set are not fool proof in a situation where a class' method is called statically from another class method where $this is defined (possibly within the same class!).