CREATE TABLE `categories` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`parent_id` int(11) NOT NULL DEFAULT '0',
`name` varchar(100) COLLATE utf8_unicode_ci NOT NULL,
`created` datetime NOT NULL,
`modified` datetime NOT NULL,
`status` enum('1','0') COLLATE utf8_unicode_ci NOT NULL DEFAULT '1' COMMENT '1:Active, 0:Inactive',
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

<?php
// Database configuration
$dbHost = "localhost";
$dbUsername = "root";
$dbPassword = "root";
$dbName = "ittutorial";

// Create database connection
$db = new mysqli($dbHost, $dbUsername, $dbPassword, $dbName);

// Check connection
if ($db->connect_error) {
die("Connection failed: " . $db->connect_error);
}
?>

<?php
// Include the database configuration file
require 'dbConfig.php';

function categoryTree($parent_id = 0, $sub_mark = ''){
global $db;
$query = $db->query("SELECT * FROM categories WHERE parent_id = $parent_id ORDER BY name ASC");

if($query->num_rows > 0){
while($row = $query->fetch_assoc()){
echo '<option value="'.$row['id'].'">'.$sub_mark.$row['name'].'</option>';
categoryTree($row['id'], $sub_mark.'---');
}
}
}

<select name="category">
<?php categoryTree(); ?>
</select>

function dashboardTiles($parent_id = 0, $sub_mark = '')
{
$db = new mysqli("localhost","itech","mahi786","itechxpert");

$query = $db->query("SELECT * FROM bis_tiles_master WHERE int_pid = $parent_id ORDER BY pki_id ASC");

if($query->num_rows > 0){
while($row = $query->fetch_assoc()){

echo '<option value="'.$row['pki_id'].'" >'.$sub_mark.$row['uvc_tile_name'].'</option>';

dashboardTiles($row['pki_id'], $sub_mark.'----');
}
}
}

<div class="form-group">
<div class="col-md-12">
<div class="form-group row">

<label for="inputKey" class="col-md-3 control-label"> Tiles Position <span class="required" aria-required="true"> * </span></label>
<div class="col-md-6">
<select class="form-control requireds selectpicker" name="tiles_position" id="tiles_position">

<option value="" selected="selected" disabled="disabled">--- Select Parent ---</option>
<option value="0" style="font-weight:bold;"> Parent </option>

<?php echo dashboardTiles(); ?>
</select>
<span class="error"><?php echo form_error('tiles_position'); ?></span>
</div>

</div>
</div>
</div>