diff --git wp-includes/compat.php wp-includes/compat.php
index cb2a559..7d99d20 100644
--- wp-includes/compat.php
+++ wp-includes/compat.php
@@ -13,25 +13,6 @@ if ( !function_exists('_') ) {
 	}
 }
 
-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( '', $chars );
-}
-
 if ( !function_exists('hash_hmac') ):
 function hash_hmac($algo, $data, $key, $raw_output = false) {
 	return _hash_hmac($algo, $data, $key, $raw_output);