Fix warnign

Zend/zend_interfaces.c

@@ -287,7 +287,7 @@ static zend_object_iterator *zend_user_it_get_new_iterator(zend_class_entry *ce,
 static int zend_implement_traversable(zend_class_entry *interface, zend_class_entry *class_type TSRMLS_DC)
 {
 	/* check that class_type is traversable at c-level or implements at least one of 'aggregate' and 'Iterator' */
-	int i;
+	zend_uint i;
 
 	if (class_type->get_iterator || (class_type->parent && class_type->parent->get_iterator)) {
 		return SUCCESS;