Index: wp-includes/compat.php
===================================================================
--- wp-includes/compat.php	(revision 0)
+++ wp-includes/compat.php	(revision 0)
@@ -0,0 +1,31 @@
+<?php
+/**
+ * WordPress implementation for PHP functions missing from older PHP versions.
+ *
+ * @package PHP
+ * @access private
+ */
+
+// Added in PHP 5.0
+
+// If gettext isn't available
+if ( !function_exists('_') ) {
+	function _($string) {
+		return $string;
+	}
+}
+
+if ( !function_exists('mb_substr') ):
+	function mb_substr( $str, $start, $length=null, $encoding=null ) {
+		// the solution below, works only for utf-8, so in case of a different
+		// charset, just use built-in substr
+		$charset = get_option( 'blog_charset' );
+		if ( !in_array( $charset, array('utf8', 'utf-8', 'UTF8', 'UTF-8') ) ) {
+			return is_null( $length )? substr( $str, $start ) : substr( $str, $start, $length);
+		}
+		// use the regex unicode support to separate the UTF-8 characters into an array
+		preg_match_all( '/./us', $str, $match );
+		$chars = is_null( $length )? array_slice( $match[0], $start ) : array_slice( $match[0], $start, $length );
+		return implode( '', $chars );
+	}
+endif;