Add values from multiple checkboxes to same row in database with jquery

I've been researching this for 2 days but have yet to find an adequate solution to my issue. Essentially I am trying to "link" database rows by ID based on multiple check boxes using jQuery, but here is where it gets a little tricky:

Background:

• I have a database table (positions) displayed as an HTML table (#ct) with check boxes next to each row.
• Up to four (4) rows can be checked at a time and "linked" to one another (but don't worry about the limit of number of boxes that can be checked)
• Linking the rows from positions SHOULD create one new row in covered_positions with columns id(PK), linked_id1, linked_id2, linked_id3, linked_id4

What is already done:

• Each checkbox already has its value assigned to the respective row id in positions
• A dropdown select box already triggers the Javascript function ctActions()

What I think is needed:

• Create an array in ctActions() of the checkbox values using something like:

  var linked_ids = [];
  
  $("#ct input[type='checkbox']:checked").each(function() {
      linked_ids.push(this.value);
  });
  

• Encode and send the data to PHP:

  var linked_ids_json = JSON.stringify(linked_ids);
  
  $.post('link-positions.php', { linked_ids_json: linked_ids_json } );
  

• In my link-positions.php:

  // Get & explode array
  $array = json_decode($_POST['linked_ids_json']);
  
  // Test whether each value is set -- Not sure if needed
  $linked_id1 = (isset($array[0]) ? $array[0] : "");
  $linked_id2 = (isset($array[1]) ? $array[1] : "");
  $linked_id3 = (isset($array[2]) ? $array[2] : "");
  $linked_id4 = (isset($array[3]) ? $array[3] : "");
  
  include('../mysqli-connect.php');
  $conn = dbConnect();
  
  $sql = "INSERT INTO `covered_positions` (
      linked_id1,
      linked_id2,
      linked_id3,
      linked_id4)
      VALUES (?, ?, ?, ?)";
  
  $stmt = $conn->prepare($sql);
  $stmt->bind_param('iiii',
              $linked_id1,
              $linked_id2,
              $linked_id3,
              $linked_id4);
  
  $stmt->execute();
  
  $conn->close();
  

But after all this, nothing happens. I'm sure there are some blatant errors in my code so I thought I would post it for some more experienced eyes.

UPDATES

• Fixed $stmt->execute to $stmt->execute() but still does not solve issue.