i have made a bootstrap accordion nav that loads data dynamically from database with php

here is the 2 tables that i use:

CREATE TABLE IF NOT EXISTS `top_tier` (
  `id` varchar(50) NOT NULL,
  `name` varchar(255) NOT NULL,
  `date_created` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `created_by` varchar(50) NOT NULL,
  `date_edited` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `status` int(11) NOT NULL DEFAULT '1',
  PRIMARY KEY (`id`)
)

CREATE TABLE IF NOT EXISTS `sub_tier` (
  `id` varchar(50) NOT NULL,
  `name` varchar(255) NOT NULL,
  `type` varchar(50) NOT NULL,
  `date_created` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `created_by` varchar(50) NOT NULL,
  `date_edited` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `edited_by` varchar(50) NOT NULL,
  `parent_id` varchar(50) NOT NULL,
  `status` int(11) NOT NULL DEFAULT '1',
  PRIMARY KEY (`id`)
)

so basically the table top_tier is the main items of the accordion nav bar. an entry in the top_tier table can have multiple children from the table sub_tier and that said entry from the sub_tier table can have children too through the column parent_id, its kinda of a hierarchy. so when i click an item from the accordion that's from the top_tier table, it'll show its children then if i click an item from that said top_tier item, it'll show its children items if it has any and so on...

here is the code for the accordion

    <div id="MainMenu">
  <div class="list-group panel">
    <?php
    $top_items =  $obj->getTopTierItems();
    foreach($top_items as $top_item) {
    extract($top_item);
    ?>

    <a href="#<?php echo $id;?>" class="strong list-group-item" data-toggle="collapse" data-parent="#MainMenu">
      <?php echo $name;?> 
      <i class="fa fa-caret-down"></i>
    </a>

    <div class="output collapse" id="<?php echo $id; ?>">
      <?php 
      $sub_items = $obj->getSubTierItems($id); 
      foreach($sub_items as $sub_item) {
        $id2 = $sub_item['id'];
        $name2 = $sub_item['name'];
        $parent_id = $sub_item['parent_id'];
      ?>
        <a href="#<?php echo $id2?>" class="strong list-group-item" data-parent="#<?php echo $parent_id; ?>">
          <?php echo $name2;?>
        </a>

        <div class="output collapse" id="<?php echo $id2; ?>">

        </div>
        <?php
        }
        ?>
    </div>

    <?php 
    }
    ?>
  </div>
</div>

here is the php that fetch the data from the table top_tier

public function getTopTierItems(){
        
        $SQL = "
            SELECT
             
            id,
            name

            FROM 
                top_tier
            WHERE
                status = 1  
        ";              
        $results = $this->db_query_listx($SQL);
        return $results;
    }

here is the php that fetch the data from the table sub_tier

public function getSubTierItems($id){
        
        $SQL = "
            SELECT
             
            id,
            name,
            parent_id

            FROM
                sub_tier
            WHERE
                status = 1
            AND
                parent_id='$id'
                        
        ";              
        $results = $this->db_query_listx($SQL);
        return $results;
    }

here is the function that controls the data from the database.

public function db_query_listx($SQL){
        global $connect;
        if ($resultset = $connect->query($SQL)) {
            if ($resultset->num_rows > 0) {
            
                $data = array();
                    while( $row = $resultset->fetch_assoc() ) {
                        $data[] = array_change_key_case($row);
                    }
                }else {
                $data = NULL;
            }
        } else {
            $data = die("<CENTER><DIV class='error'>ERROR: " . $connect->error . "<BR> ( $SQL ) <HR>
                             Please report this to <a href='#'>webmaster</a>.</DIV></CENTER>");
        }
        $resultset->close();
        return $data;
    }