PHPで漢字を置き換える方法

$username = "linshouyue";
echo substr_replace($username,'****','3','4');

/**     * *替换中文汉字
     * @author 月月
     */
    function substr_cut($user_name){
        $strlen     = mb_strlen($user_name, 'utf-8');
        $firstStr     = mb_substr($user_name, 0, 1, 'utf-8');
        $lastStr     = mb_substr($user_name, -1, 1, 'utf-8');
        return $strlen == 2 ? $firstStr . str_repeat('*', mb_strlen($user_name, 'utf-8') - 1) : $firstStr . str_repeat("*", $strlen - 2) . $lastStr;
    }