Rename 'kohana' to 'system' to conform to the Kohana filesystem layout. I'm comfortable with us not clearly drawing the distinction about the fact that it's Kohana.

system/core/utf8/substr_replace.php

@@ -0,0 +1,22 @@
+<?php defined('SYSPATH') OR die('No direct access allowed.');
+/**
+ * utf8::substr_replace
+ *
+ * @package    Core
+ * @author     Kohana Team
+ * @copyright  (c) 2007 Kohana Team
+ * @copyright  (c) 2005 Harry Fuecks
+ * @license    http://www.gnu.org/licenses/old-licenses/lgpl-2.1.txt
+ */
+function _substr_replace($str, $replacement, $offset, $length = NULL)
+{
+	if (utf8::is_ascii($str))
+		return ($length === NULL) ? substr_replace($str, $replacement, $offset) : substr_replace($str, $replacement, $offset, $length);
+
+	$length = ($length === NULL) ? utf8::strlen($str) : (int) $length;
+	preg_match_all('/./us', $str, $str_array);
+	preg_match_all('/./us', $replacement, $replacement_array);
+
+	array_splice($str_array[0], $offset, $length, $replacement_array[0]);
+	return implode('', $str_array[0]);
+}