[php] Adjust #required params for subclass methods

If the same method name is implemented in a parent class then the
subclass can't have more required parameters than that or else we
get a compatibility error when the module is loaded.

The testsuite wasn't catching this problem because it was no longer
trying to load the modules for testcases without _runme.php, because
the mechanism to do that relied on there being a generated .php
wrapper, which we no longer have by default.  Fix that to provide a
regression test for this fix.

See #2151

Examples/test-suite/php/director_stl_runme.php

@@ -26,7 +26,7 @@ class MyFoo extends Foo {
     return $v;
   }
 
-  function vsecond($v1, $v2) {
+  function vsecond($v1, $v2 = NULL) {
     return $v2;
   }
 }