php怎么随机生成不重复字母

$activecode = array();
$len = 12;
$str = ‘abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ’;
for($i=0;$i<5000;$i++){
    $code = str_shuffle($str);
    $code = substr($code, 0, $len);
    $num = strval($i);
    $num_len = strlen($num);
    $p = -1;
    for($j=0;$j<$num_len;$j++){
        $p = rand($p+1, ($len-1)-($num_len-$j-1));
        $code[$p] = $num[$j];
    }
    $activecode[] = $code;
}
print_r($activecode);