diff --git a/wp-includes/compat.php b/wp-includes/compat.php
index 3531017e5..e1e0fa532 100644
--- a/wp-includes/compat.php
+++ b/wp-includes/compat.php
@@ -12,3 +12,22 @@ if ( !function_exists('_') ) {
 		return $string;
 	}
 }
+
+if ( !function_exists('mb_substr') ):
+	function mb_substr( $str, $start, $length=null, $encoding=null ) {
+		return _mb_substr($str, $start, $length, $encoding);
+	}
+endif;
+
+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 );
+}