convert file to utf8

function convert_to_utf8($fileName) {
    $fileName = str_replace('/', '\\\\', $fileName);
    $encoding = mb_detect_encoding($fileName);
    
    if ($encoding != 'UTF-8') {
        if ($encoding == 'GB18030') {
            $encoding = 'GBK';
        }
        echo 'The file is not utf8, encoding is ' . $encoding . '<br>';
        $str = file_get_contents($fileName);
        $str = mb_convert_encoding($str, 'UTF-8', $encoding);
        file_put_contents($fileName, $str);
    }
}