比较准确地检测字符的编码是否是UTF8

function valid_1byte($char) {
        if(!is_int($char)) return false;
        return ($char & 0x80) == 0x00;
    }
   
    function valid_2byte($char) {
        if(!is_int($char)) return false;
        return ($char & 0xE0) == 0xC0;
    }

    function valid_3byte($char) {
        if(!is_int($char)) return false;
        return ($char & 0xF0) == 0xE0;
    }

    function valid_4byte($char) {
        if(!is_int($char)) return false;
        return ($char & 0xF8) == 0xF0;
    }
   
    function valid_nextbyte($char) {
        if(!is_int($char)) return false;
        return ($char & 0xC0) == 0x80;
    }
   
    function valid_utf8($string) {
        $len = strlen($string);
        $i = 0;   
        while( $i < $len ) {
            $char = ord(substr($string, $i++, 1));
            if(valid_1byte($char)) {    // continue
                continue;
            } else if(valid_2byte($char)) { // check 1 byte
                if(!valid_nextbyte(ord(substr($string, $i++, 1))))
                    return false;
            } else if(valid_3byte($char)) { // check 2 bytes
                if(!valid_nextbyte(ord(substr($string, $i++, 1))))
                    return false;
                if(!valid_nextbyte(ord(substr($string, $i++, 1))))
                    return false;
            } else if(valid_4byte($char)) { // check 3 bytes
                if(!valid_nextbyte(ord(substr($string, $i++, 1))))
                    return false;
                if(!valid_nextbyte(ord(substr($string, $i++, 1))))
                    return false;
                if(!valid_nextbyte(ord(substr($string, $i++, 1))))
                    return false;
            } // goto next char
        }
        return true; // done
    }