php语法如何随机生成字符串（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);