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.php

@@ -0,0 +1,75 @@
+<?php defined('SYSPATH') OR die('No direct access allowed.');
+/**
+ * utf8::substr
+ *
+ * @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($str, $offset, $length = NULL)
+{
+	if (SERVER_UTF8)
+		return ($length === NULL) ? mb_substr($str, $offset) : mb_substr($str, $offset, $length);
+
+	if (utf8::is_ascii($str))
+		return ($length === NULL) ? substr($str, $offset) : substr($str, $offset, $length);
+
+	// Normalize params
+	$str    = (string) $str;
+	$strlen = utf8::strlen($str);
+	$offset = (int) ($offset < 0) ? max(0, $strlen + $offset) : $offset; // Normalize to positive offset
+	$length = ($length === NULL) ? NULL : (int) $length;
+
+	// Impossible
+	if ($length === 0 OR $offset >= $strlen OR ($length < 0 AND $length <= $offset - $strlen))
+		return '';
+
+	// Whole string
+	if ($offset == 0 AND ($length === NULL OR $length >= $strlen))
+		return $str;
+
+	// Build regex
+	$regex = '^';
+
+	// Create an offset expression
+	if ($offset > 0)
+	{
+		// PCRE repeating quantifiers must be less than 65536, so repeat when necessary
+		$x = (int) ($offset / 65535);
+		$y = (int) ($offset % 65535);
+		$regex .= ($x == 0) ? '' : '(?:.{65535}){'.$x.'}';
+		$regex .= ($y == 0) ? '' : '.{'.$y.'}';
+	}
+
+	// Create a length expression
+	if ($length === NULL)
+	{
+		$regex .= '(.*)'; // No length set, grab it all
+	}
+	// Find length from the left (positive length)
+	elseif ($length > 0)
+	{
+		// Reduce length so that it can't go beyond the end of the string
+		$length = min($strlen - $offset, $length);
+
+		$x = (int) ($length / 65535);
+		$y = (int) ($length % 65535);
+		$regex .= '(';
+		$regex .= ($x == 0) ? '' : '(?:.{65535}){'.$x.'}';
+		$regex .= '.{'.$y.'})';
+	}
+	// Find length from the right (negative length)
+	else
+	{
+		$x = (int) (-$length / 65535);
+		$y = (int) (-$length % 65535);
+		$regex .= '(.*)';
+		$regex .= ($x == 0) ? '' : '(?:.{65535}){'.$x.'}';
+		$regex .= '.{'.$y.'}';
+	}
+
+	preg_match('/'.$regex.'/us', $str, $matches);
+	return $matches[1];
+}