<div class="tree">
  <ul>
    <li>
      <div><input type="checkbox">181210-1-105547-1<br/> <button> Test Btn </button></div>
      <ul>

<?php
    $host = 'localhost';
    $name = 'mlm';
    $user = 'root';
    $pass = '';

    $dsn = 'mysql:host=' .$host .';dbname=' .$name;
    $options = array(
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
        PDO::ATTR_PERSISTENT => true
    );
    $conn = new PDO($dsn, $user, $pass, $options);

    function displayChildren($parent) {
        global $conn;

        $stmt = $conn->prepare('SELECT * FROM accounts WHERE sponsorUpline = ?');
        $stmt->bindValue(1, $parent);
        $stmt->execute();
        while($row = $stmt->fetch(PDO::FETCH_OBJ)) {
            echo '<li></div><input type="checkbox">' .$row->serialNumber .'</div></li>';
            displayChildren($row->serialNumber);
        }
  }

    displayChildren('181210-1-105547-1');

?>

      </ul>
    </li>
  </ul>
</div>