hello. i got this error and i have no idea why or what is going on. could someone explain in layman's term please and help me out.

Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'event_id' cannot be null' in /home/dhsbnet/public_html/ems/add_guest.php:36 Stack trace: #0 /home/dhsbnet/public_html/ems/add_guest.php(36): PDOStatement->execute() #1 {main} thrown in /home/dhsbnet/public_html/ems/add_guest.php on line 36

code being referred to, line 36 is $guests->execute() :

if(isset($_SESSION['sess_user_id']))
{
  if(isset($_POST['save']))
  {
    require "connection.php";
    $name = $_POST['guest-name'];
    $event = $_POST['event'];
    $session = $_SESSION['sess_user_id'];
    $status = "";
    $status .= "0";

    $guests = $dbh->prepare("INSERT INTO guest(guser_id,guest_name,event_id,status) VALUES (?,?,?,?)");

    for($i = 0; $i < count($_POST['guest-name']); $i++)
    {
      if(trim($_POST['guest-name'][$i]) !== '')
      {
        $guests->bindParam(1, $session, PDO::PARAM_INT);
        $guests->bindParam(2, $name[$i], PDO::PARAM_STR);
        $guests->bindParam(3, $event, PDO::PARAM_INT);
        $guests->bindParam(4, $status, PDO::PARAM_INT);
        $guests->execute();

        if($guests->rowCount() > 0)
        {
          echo "<script>window.location.href = 'guest_list.php';</script>";
        } 
      }
    }
  }
}

<div class="dropdown">
    <label>Event : <label/>
    <select name="event" id="dd-event" required>
      <option value="0" selected>Event</option>
      <?php foreach($retrieve as $r): ?>
      <option value="<?=$r['event_id']?>"><?=$r['event_name']?></option>
      <?php endforeach ?>
    </select>
  </div>

in db event_id column is int, not null, not auto incremented. TIA!

Check your generated HTML for the dropdown. It says you are trying to insert an event without an ID. Probably, one of the options does not have a value. If it is, check your POST array for the value of event.

but i checked it. i changed this:

<option value="<?=$r['event_id']?>"><?=$r['event_name']?></option>

to

<option value="<?=$r['event_id']?>"><?=$r['event_id']?></option>

so i could c what the event id and it does show the id.

column 'event_id' cannot be null

The error says it is. Check the $_POST array.

i used var_dump and it is null. coz i always automatically assume its a php problem i didn't check my html so i did and found that the drop down wasn't in the form tag.