[php] Allow testing if an object is SWIG-wrapped

Since the switch to wrapping classes using PHP's C API, we now
internally need to be able to tell if a PHP object is of or derived
from a class that is wrapped by SWIG so we know if we can offset the
zend_object pointer to get to the swig_object_wrapper.  If we try to
do this to an object which isn't wrapped by SWIG then we invoke C/C++
undefined behaviour (and typically get a segmentation fault).

This check is implemented by having a SWIG\wrapped empty interface which
we make all SWIG-wrapped classes implement simply so we can test for it
to detect such classes.

Fixes #2125

Examples/test-suite/php/overload_simple_runme.php

@@ -187,8 +187,9 @@ check("' 11 '", 'char *');
 
 # Check TypeError is thrown when the wrong type is passed.
 check("array()", NULL);
-# FIXME: These need fixing
-#check("function(){}", NULL);
-#check("new stdClass()", NULL);
+check("function(){}", NULL);
+check("new stdClass()", NULL);
+class NotASwigWrappedClass { };
+check("new NotASwigWrappedClass()", NULL);
 
 check::done();