php过滤掉无法识别的问号字符解决方案

$str = file_get_contents('example_text.txt');
echo $str;

function filter_utf8_char($ostr){
    preg_match_all('/[\x{FF00}-\x{FFEF}
------解决方案--------------------
\x{0000}-\x{00ff}
------解决方案--------------------
\x{4e00}-\x{9fff}]+/u', $ostr, $matches);
    $str = join('', $matches[0]);
    if($str==''){   //含有特殊字符需要逐個處理
        $returnstr = '';
        $i = 0;
        $str_length = strlen($ostr);
        while ($i<=$str_length){
            $temp_str = substr($ostr, $i, 1);
            $ascnum = Ord($temp_str);
            if ($ascnum>=224){
                $returnstr = $returnstr.substr($ostr, $i, 3);
                $i = $i + 3;
            }elseif ($ascnum>=192){
                $returnstr = $returnstr.substr($ostr, $i, 2);
                $i = $i + 2;
            }elseif ($ascnum>=65 && $ascnum<=90){
                $returnstr = $returnstr.substr($ostr, $i, 1);
                $i = $i + 1;
            }elseif ($ascnum>=128 && $ascnum<=191){ // 特殊字符
                $i = $i + 1;
            }else{
                $returnstr = $returnstr.substr($ostr, $i, 1);
                $i = $i + 1;
            }
        }
        $str = $returnstr;
        preg_match_all('/[\x{FF00}-\x{FFEF}
------解决方案--------------------
\x{0000}-\x{00ff}
------解决方案--------------------
\x{4e00}-\x{9fff}]+/u', $str, $matches);
        $str = join('', $matches[0]);
    }
    return $str;
}