PHP生成8位不重復字符串

function randString():string
{
    $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;
}

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