file_put_contents 中文乱码问题

/**
 * 将非GBK字符集的编码转为GBK
 *
 * @param mixed $mixed 源数据
 *
 * @return mixed GBK格式数据
 */
function charsetToGBK($mixed)
{
    if (is_array($mixed)) {
        foreach ($mixed as $k => $v) {
            if (is_array($v)) {
                $mixed[$k] = charsetToGBK($v);
            } else {
                $encode = mb_detect_encoding($v, array('ASCII', 'UTF-8', 'GB2312', 'GBK', 'BIG5'));
                if ($encode == 'UTF-8') {
                    $mixed[$k] = iconv('UTF-8', 'GBK', $v);
                }
            }
        }
    } else {
        $encode = mb_detect_encoding($mixed, array('ASCII', 'UTF-8', 'GB2312', 'GBK', 'BIG5'));
        //var_dump($encode);
        if ($encode == 'UTF-8') {
            $mixed = iconv('UTF-8', 'GBK', $mixed);
        }
    }
    return $mixed;
}


/**
 * 将非UTF-8字符集的编码转为UTF-8
 *
 * @param mixed $mixed 源数据
 *
 * @return mixed utf-8格式数据
 */
function charsetToUTF8($mixed)
{
    if (is_array($mixed)) {
        foreach ($mixed as $k => $v) {
            if (is_array($v)) {
                $mixed[$k] = charsetToUTF8($v);
            } else {
                $encode = mb_detect_encoding($v, array('ASCII', 'UTF-8', 'GB2312', 'GBK', 'BIG5'));
                if ($encode == 'EUC-CN') {
                    $mixed[$k] = iconv('GBK', 'UTF-8', $v);
                }
            }
        }
    } else {
        $encode = mb_detect_encoding($mixed, array('ASCII', 'UTF-8', 'GB2312', 'GBK', 'BIG5'));
        if ($encode == 'EUC-CN') {
            $mixed = iconv('GBK', 'UTF-8', $mixed);
        }
    }
    return $mixed;
}

<?php   file_put_contents("./sitemap.txt", charsetToGBK($g_txt));?>