We don't want ReflectionMethod::invoke() to simply ignore its first argument,
if the method to invoke is a static method. Instead we match its ZPP with
that of ReflectionMethod::invokeArgs(). Furthermore, we apply the DRY
principle by factoring out the code to a common helper function to prevent
inadvertent future divergence of the implementations of both methods.
As can be seen from the necessity to adapt some test cases, this causes a
BC break for some pathological cases. Therefore we apply this patch to PHP
7.1 only, which is still in beta phase.
