PHP generates 8-digit non-repeating strings

function randString() {
        $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;
}

echo randString().'
';
echo randString().'
';
echo randString().'
';

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