PHP converts the encoding of all files in the folder

<?php
/**
 * 把一个文件夹里的文件全部转码 只能转一次 否则全部变乱码
 * @param string $filename
 */
function iconv_file($filename,$input_encoding=&#39;gbk&#39;,$output_encoding=&#39;utf-8&#39;)
{
    if(file_exists($filename))
    {
        if(is_dir($filename))
        {
            foreach (glob("$filename/*") as $key=>$value)
            {
                iconv_file($value);
            }
        }
        else
        {
            $contents_before = file_get_contents($filename);
            /*$encoding = mb_detect_encoding($contents_before,array(&#39;CP936&#39;,&#39;ASCII&#39;,&#39;GBK&#39;,&#39;GB2312&#39;,&#39;UTF-8&#39;));
            echo $encoding;
            if($encoding==&#39;UTF-8&#39;)  mb_detect_encoding函数不工作
            {
                return;
            }*/
            $contents_after = iconv($input_encoding,$output_encoding,$contents_before);
            file_put_contents($filename, $contents_after);
        }
    }
    else
    {
        echo &#39;参数错误&#39;;
        return false;
    }
}
iconv_file(&#39;./test&#39;);
?>