How to make side - category system - php

Question

Soo, I need to make a side category system but I am litlle bit confused about it. Can someone help me out ?
Here is my PHP
<?php
// Conncetion to database
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "shop";
// Create connection
$conn = mysqli_connect($servername, $username, $password, $dbname);
mysqli_set_charset($conn,"utf8");
// Check connection
if (!$conn) {
die("Connection failed: " . mysqli_connect_error());
}
// SQL Query
$sql = "SELECT category_id, category_name, category_link, parent_id, sort_order FROM category2 ORDER BY parent_id, sort_order, category_name";
$result = mysqli_query($conn, $sql);
//Create a multidimensional array to hold a list of category and parent category
$category = array(
'categories' => array(),
'parent_cats' => array()
);
//Build the array lists with data from the category table
while ($row = mysqli_fetch_assoc($result)) {
//creates entry into categories array with current category id ie. $categories['categories'][1]
$category['categories'][$row['category_id']] = $row;
//creates entry into parent_cats array. parent_cats array contains a list of all categories with children
$category['parent_cats'][$row['parent_id']][] = $row['category_id'];
}
function buildCategory($parent, $category) {
//Code HERE...
}
echo buildCategory(0, $category);
Soo, when you are on homepage then I just need every parent elements with "category_id = 0", then if someone click any parent category should shows up his children, but these childrens has another childrens and etc. etc. but it needs to show always parent elements with "category_id = 0".
I found live example here: https://papiernictvotriomat.sk
Also posting here image of my database:

Answer 1

You can change your database a little and add a field with all the way up to the root category separated by comma, but this is not a "good option" in the relational world.
The simples way is just get all categories you have - like you're doing - and create the hierarchy with a recursive function:
<?php
$conn = mysqli_connect('localhost', 'root', 'l2mago666', 'stack');
# This ensure the root one come first
$res = mysqli_query($conn, 'SELECT * FROM categories ORDER BY parent');
$categories = [];
while($rs = mysqli_fetch_assoc($res)) {
if (!$rs['parent'])
$categories[$rs['id']] = $rs;
else
$rows[] = $rs;
}
function lookup(&$set, $r) {
foreach($set as &$s) {
if ($s['id'] == $r['parent']) {
if(!isset($s['childs']))
$s['childs'] = [];
$s['childs'][$r['id']] = $r;
} else {
if(isset($s['childs']))
lookup($s['childs'], $r);
}
}
}
foreach($rows as $r)
lookup($categories, $r);
print_r($categories);
This should help you create the HTML, of course, with another recursive function.