如何使用PHP / MySQL基于表的行创建特定大小的随机数？

I want to create a specific-sized random "key" for every row in the database table. For example, for the "id" or row (auto-increment ID) "20", the column "key" will have "xEd456" and for row 21, it will be e.g. "rYU875". However, I also want to make sure there is no way there is a duplicate "key" in the table. I prefer to have 6-char "key". How do I achieve this?

That means you need a alphanumeric key, see the code

while(true)
  {
  $str="";
  for($i=0;$i<6;$i++)
    {
    $rnd=rand(1,16);
    if(intval(fmod($rnd, 2)))
    $a=rand(48,57);
    else
    $a=rand(65,90);
    $str .= chr($a);
    }
  // skipping the mysql connection functions
  $sql=mysql_query(“select user_id from user_master_table where user_id_alias='".$str."'");
  $rs=mysql_fetch_array($sql);
  if($rs["user_id"]=="")
    break;
  }

You are free to change the loop value and random number range.