In this article we will create a multi-level category menu using PHP and mysql, by calling a recursive function, we can display infinite level of categories and subcategories.

Follow step by step instruction:

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1) Create Database:
You need to create a Database 'multi_level' and a table 'category' with 3 columns as following:

category_id : Primary key.
category_name : Name of the category.
parent_id : It is the parent to corresponding category_id. For root category parent_id is 0.

2) Create Database connection:

<?php
DEFINE('DB_HOST','localhost');
DEFINE('DB_USER','root');
DEFINE('DB_NAME','multi_level');
DEFINE('DB_PASS','');
 

$con = mysqli_connect(DB_HOST, DB_USER, DB_PASS, DB_NAME) or 
      die('could not connect: '. mysqli_connect_error() );

// Check connection
if (mysqli_connect_errno()) {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
?>

3) Create mysql query to fetch data:

<?php 
$sql = "SELECT category_id, category_name, parent_id FROM category ORDER BY parent_id, category_id";
$results = mysqli_query($con,$sql) or die(mysqli_error()) ;
if($results)
{
    while($result = mysqli_fetch_array($results))
    {
        $category['categories'][$result['category_id']] = $result; 
        $category['parent_cats'][$result['parent_id']][] = $result['category_id']; 
    }
}
?>

4) Create Recursive function to display Category Hierarchy:

<?php 
function getCategories($parent, $category) 
{
    $html = "";
    if (isset($category['parent_cats'][$parent]))
    {
        $html .= "<ul>\n";
        foreach ($category['parent_cats'][$parent] as $cat_id)
        {
            if (!isset($category['parent_cats'][$cat_id]))
            {
              $html .= "<li>".$category['categories'][$cat_id]['category_name']."</li> \n";
            }
            if (isset($category['parent_cats'][$cat_id]))
            {
              $html .= "<li>". $category['categories'][$cat_id]['category_name'] . " \n";
              $html .= getCategories($cat_id, $category);
              $html .= "</li> \n";
            }
        }
        $html .= "</ul> \n";
    }
    return $html;
}
?>

5) Now call Recursive function to display all Category in Hierarchy:

<?php echo $data['category'] = getCategories(0, $category);?>

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