WordPress/wp-includes/compat.php

<?php
/**
 * WordPress implementation for PHP functions either missing from older PHP versions or not included by default.
 *
 * @package PHP
 * @access private
 */

// If gettext isn't available
if ( !function_exists('_') ) {
	function _($string) {
		return $string;
	}
}

if ( !function_exists('mb_substr') ):
	function mb_substr( $str, $start, $length=null, $encoding=null ) {
		return _mb_substr($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 );
}