为每个MySQL行调用一个函数

I have a standard MySQL database, with around 60 rows (as in user accounts). When I first made it I made the mistake of making session IDs the same as the simple account ID, now I want to fix my mistake and I am obviously not going to go through 60 rows to reset them different secure session IDs, so I am writing this function:

function generate_sessionid(){
    return bin2hex(openssl_random_pseudo_bytes(32));
}

function assign_all_sessionids(){
    $sessionid = generate_sessionid();

    $conn = sql_connect();
    $result = mysqli_query($conn, "UPDATE accounts SET sessionid='$sessionid' WHERE 1");
    sql_disconnect($conn);
}
assign_all_sessionids();

Problem: Every account in the database gets the same random session ID as the rest. How do I make it recall the function for each row in order to allow it to be random for each row?

Try get user's count from DB and simply execute it N times

function assign_all_sessionids(){

  $conn = sql_connect();

  // getting users count
  // here just change 'id' to your id parameter
  $result = mysqli_query($conn, "SELECT id FROM accounts"); 
  $arr = $result->fetch_array(MYSQLI_NUM);

  // executing N times
  for($i = 0; $i < $result->num_rows; $i++){
     $sessionid = generate_sessionid();
  // here just change 'id' to your id parameter again
     mysqli_query($conn, "UPDATE accounts SET sessionid='$sessionid' WHERE `id`=".$arr[$i]);
  }

sql_disconnect($conn);
}

You can do what you want by first setting all the session ids to NULL:

UPDATE accounts
    SET sessionid = NULL;

Then, inside the loop:

UPDATE accounts
    SET sessionid = '$sessionid'
    WHERE sessionid IS NOT NULL
    LIMIT 1;

Normally you don't want to execute queries in a loop, however in this case you need to get all of the current unique identifiers, loop and generate a new identifier and then update one:

function assign_all_sessionids(){
    $conn = mysqli_connect('whatever...');
    $select = mysqli_query($conn, "SELECT sessionid FROM accounts");

    while(list($id) = mysqli_fetch_assoc($select)) {
        $sessionid = generate_sessionid();
        $update = mysqli_query($conn, "UPDATE accounts SET sessionid='$sessionid' WHERE sessionid='$id'");
    }
}