php生成8位不重复字符串

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().'<br>';
echo randString().'<br>';
echo randString().'<br>';

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