how to display an hierarchical structured database in tree view?

Question

For some reasons need to display data from a mysql database in 2 different tree views.

Example

Tree view 1 (using list tag):

<li>level1 (Root)
	<ul>
		<li>level2</li>
		<li>level2
			<ul>
				<li>level3</li>
				<li>level3
					<ul>
						<li>level4</li>
						<li>level4</li>
					</ul>
				</li>
			</ul>
		</li>
		<li>level2
			<ul>
				<li>level3</li>
				<li>level3
					<ul>
						<li>level4</li>
						<li>level4</li>
					</ul>
				</li>
			</ul>
		</li>
	</ul>
</li>

Tree view 2:

I'm currently working on this php script:

<?php
$servername = "localhost";
$username = "root";
$password = "psswd_here";
$dbname = "db_here";

$conn = new mysqli($servername, $username, $password, $dbname);

if ($conn->connect_error)
{
    die("Connection failed: " . $conn->connect_error);
}

$data = array();
$index = array();
$sql2 = "SELECT * FROM categories ORDER BY name";
$result2 = $conn->query($sql2);

if ($result2->num_rows > 0)
{
        while ($row2 = $result2->fetch_assoc())
        {
                $id = $row2["id"];
                $parent_id = $row2["parent_id"] === NULL ? "NULL" : $row2["parent_id"];
                $data[$id] = $row2;
                $index[$parent_id][] = $id;
        }
}
else
{
    echo "0 results";
}

$conn->close();

function display_child_nodes($parent_id, $level)
{
    global $data, $index;
    $parent_id = $parent_id === NULL ? "NULL" : $parent_id;
    if (isset($index[$parent_id]))
        {
        foreach ($index[$parent_id] as $id)
                {
            echo str_repeat("-", $level) . $data[$id]["name"] . "<br>";
            display_child_nodes($id, $level + 1);
        }
    }
}

echo '<ul>';
display_child_nodes(NULL, 0);
echo '</ul>';
?>

OUTPUT:

Electronics
-Cameras and Photography
--Accessories
--Camcorders
--Digital Cameras
-Cell Phones and Accessories
--Accessories
--Cell Phones
--Smartphones
-Computers and Tablets
--Desktops
--Laptops
--Netbooks
--Tablets
---Android
---iPad
-TV and Audio
--Home Audio
--Speakers and Subwoofers
--Televisions
---CRT
---LCD
---LED
---Plasma

Here's the database schema..

-- phpMyAdmin SQL Dump
-- version 4.4.12
-- http://www.phpmyadmin.net
--
-- Host: 127.0.0.1
-- Generation Time: Oct 21, 2015 at 09:16 AM
-- Server version: 5.6.25
-- PHP Version: 5.6.11

SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";
SET time_zone = "+00:00";


/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!40101 SET NAMES utf8mb4 */;

--
-- Database: `binary`
--

-- --------------------------------------------------------

--
-- Table structure for table `categories`
--

CREATE TABLE IF NOT EXISTS `categories` (
  `id` int(10) NOT NULL,
  `parent_id` int(10) DEFAULT NULL,
  `name` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL
) ENGINE=InnoDB AUTO_INCREMENT=25 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

--
-- Dumping data for table `categories`
--

INSERT INTO `categories` (`id`, `parent_id`, `name`) VALUES
(1, NULL, 'Electronics'),
(2, 1, 'Cameras and Photography'),
(3, 1, 'Computers and Tablets'),
(4, 1, 'Cell Phones and Accessories'),
(5, 1, 'TV and Audio'),
(6, 2, 'Digital Cameras'),
(7, 2, 'Camcorders'),
(8, 2, 'Accessories'),
(9, 3, 'Laptops'),
(10, 3, 'Desktops'),
(11, 3, 'Netbooks'),
(12, 3, 'Tablets'),
(13, 4, 'Cell Phones'),
(14, 4, 'Smartphones'),
(15, 4, 'Accessories'),
(16, 5, 'Televisions'),
(17, 5, 'Home Audio'),
(18, 5, 'Speakers and Subwoofers'),
(19, 16, 'CRT'),
(20, 16, 'LCD'),
(21, 16, 'LED'),
(22, 16, 'Plasma'),
(23, 12, 'Android'),
(24, 12, 'iPad');

--
-- Indexes for dumped tables
--

--
-- Indexes for table `categories`
--
ALTER TABLE `categories`
  ADD PRIMARY KEY (`id`);

--
-- AUTO_INCREMENT for dumped tables
--

--
-- AUTO_INCREMENT for table `categories`
--
ALTER TABLE `categories`
  MODIFY `id` int(10) NOT NULL AUTO_INCREMENT,AUTO_INCREMENT=25;
/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
/*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */;
/*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;

Answer 1

See my code below-

you just need to put here css for your exact design

$servername = "localhost";
$username = "root";
$password = "psswd_here";
$dbname = "db_here";

$conn = new mysqli($servername, $username, $password, $dbname);

if ($conn->connect_error)
{
    die("Connection failed: " . $conn->connect_error);
}

function getCategory($parent_id){
    global $conn;
    $sql2 = "SELECT * FROM categories WHERE parent_id='".$parent_id."' ORDER BY name";
    $result2 = $conn->query($sql2);

    if (mysqli_num_rows($result2) > 0)
    {
        echo "<ul>";
        while ($row2 = mysqli_fetch_object($result2))
        {
            echo "<li>".$row2->name."</li>";
            getCategory($row2->id);     
        }
        echo "</ul>";
    }
}


//Set Parent id
$parent_id = 0;
getCategory($parent_id);

Note: Root category parent_id is 0