php utf8ToGBK

function utf8ToGBK($c) {
        $str = '';
        if ($c < 0x80) {
            $str .= $c;
        } elseif ($c < 0x800) {
            $str .= chr(0xC0 | $c >> 6);
            $str .= chr(0x80 | $c & 0x3F);
        } elseif ($c < 0x10000) {
            $str .= chr(0xE0 | $c >> 12);
            $str .= chr(0x80 | $c >> 6 & 0x3F);
            $str .= chr(0x80 | $c & 0x3F);
        } elseif ($c < 0x200000) {
            $str .= chr(0xF0 | $c >> 18);
            $str .= chr(0x80 | $c >> 12 & 0x3F);
            $str .= chr(0x80 | $c >> 6 & 0x3F);
            $str .= chr(0x80 | $c & 0x3F);
        }
        return iconv('UTF-8', 'GB2312', $str);
    }