生成固定长随机码

function R() {

    $code = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';

    $rand = $code[rand(0,25)]

            .strtoupper(dechex(date('m')))

                .date('d').substr(time(),-5)

                    .substr(microtime(),2,5)

                        .sprintf('%02d',rand(0,99));

    for(

        $a = md5( $rand, true ),

        $s = '0123456789ABCDEFGHIJKLMNOPQRSTUV',

        $d = '',

        $f = 0;

        $f < 8;

        $g = ord( $a[ $f ] ),

        $d .= $s[ ( $g ^ ord( $a[ $f + 8 ] ) ) - $g & 0x1F ],

        $f++

    );

    return $d;

}