In RINIT: internal_function.handler; orig->internal_function.handler = zif_xdebug_var_dump; XG(var_dump_overloaded) = 1; } ]]> In ZEND_MODULE_POST_ZEND_DEACTIVATE_D: internal_function.handler = XG(orig_var_dump_func); }]]> And %xdebug_var_dump()% is just defined as a normal function.