The object *may* be modified by the function that is invoked on it (that function will have access to it via $this).
This is a pointer to an object on which the function is invoked. This should be NULL if a global function is called. If it's not NULL (i.e. it points to an object), the function_table argument is ignored, and instead taken from the object's hash. The object *may* be modified by the function that is invoked on it (that function will have access to it via $this). If for some reason you don't want that to happen, send a copy of the object instead.