Filter utf8 characters that exceed three bytes, or non-utf8 characters

function filterUtf8($str)
    {
        /*utf8 编码表：
        * Unicode符号范围           | UTF-8编码方式
        * u0000 0000 - u0000 007F   | 0xxxxxxx
        * u0000 0080 - u0000 07FF   | 110xxxxx 10xxxxxx
        * u0000 0800 - u0000 FFFF   | 1110xxxx 10xxxxxx 10xxxxxx
        *
        */
        $re = '';
        $str = str_split(bin2hex($str), 2);

        $mo =  1<<7;
        $mo2 = $mo | (1 << 6);
        $mo3 = $mo2 | (1 << 5);         //三个字节
        $mo4 = $mo3 | (1 << 4);          //四个字节
        $mo5 = $mo4 | (1 << 3);          //五个字节
        $mo6 = $mo5 | (1 << 2);          //六个字节


        for ($i = 0; $i < count($str); $i++)
        {
            if ((hexdec($str[$i]) & ($mo)) == 0)
            {
                $re .=  chr(hexdec($str[$i]));
                continue;
            }

            //4字节 及其以上舍去
            if ((hexdec($str[$i]) & ($mo6) )  == $mo6)
            {
                $i = $i +5;
                continue;
            }

            if ((hexdec($str[$i]) & ($mo5) )  == $mo5)
            {
                $i = $i +4;
                continue;
            }

            if ((hexdec($str[$i]) & ($mo4) )  == $mo4)
            {
                $i = $i +3;
                continue;
            }

            if ((hexdec($str[$i]) & ($mo3) )  == $mo3 )
            {
                $i = $i +2;
                if (((hexdec($str[$i]) & ($mo) )  == $mo) &&  ((hexdec($str[$i - 1]) & ($mo) )  == $mo)  )
                {
                    $r = chr(hexdec($str[$i - 2])).
                        chr(hexdec($str[$i - 1])).
                        chr(hexdec($str[$i]));
                    $re .= $r;
                }
                continue;
            }



            if ((hexdec($str[$i]) & ($mo2) )  == $mo2 )
            {
                $i = $i +1;
                if ((hexdec($str[$i]) & ($mo) )  == $mo)
                {
                    $re .= chr(hexdec($str[$i - 1])) . chr(hexdec($str[$i]));
                }
                continue;
            }
        }
        return $re;
    }