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Complete Guide to Implementing Tree Structure with Infinite Categories in PHP

gitbox 2025-07-28

Introduction

In web application development, there are often requirements for multi-level category displays, such as product categories, article classifications, or permission menus. Infinite categories, which support any level of nested structure, are an ideal solution for such needs. In this article, we will detail how to build an efficient infinite category tree structure using PHP.

Data Structure Design

To implement infinite categories, we first need to design a reasonable database table structure. Typically, a simple self-referencing table is used to store category data.


CREATE TABLE categories (
  id INT PRIMARY KEY AUTO_INCREMENT,
  parent_id INT NOT NULL,
  name VARCHAR(50) NOT NULL
);

The id field is a unique identifier for each category, and the parent_id field represents the parent category. The top-level categories typically have a parent_id of 0, while the name field stores the category name.

Querying Category Data

Before displaying the tree structure, we need to fetch the category data from the database and use recursion to generate the nested relationships:


function getCategories($parent_id = 0) {
  $categories = [];
  $query = "SELECT id, name FROM categories WHERE parent_id = :parent_id";
  $stmt = $pdo->prepare($query);
  $stmt->execute(['parent_id' => $parent_id]);

  while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
    $categories[$row['id']] = [
      'id' => $row['id'],
      'name' => $row['name'],
      'children' => getCategories($row['id'])
    ];
  }
  return $categories;
}

The code above recursively builds the complete category tree, with each category containing its subcategories' information.

Example of calling the function:


$categories = getCategories();

Displaying the Tree Structure

Once we have the category data, we need to display it as a tree structure on the web page. This can be achieved using HTML unordered list elements, and recursion is used to render the structure:


function renderCategories($categories) {
  echo '<ul>';
  foreach ($categories as $category) {
    echo '<li>' . $category['name'] . '</li>';
    if (!empty($category['children'])) {
      echo '<ul>';
      renderCategories($category['children']);
      echo '</ul>';
    }
  }
  echo '</ul>';
}

renderCategories($categories);

This function recursively displays the nested categories using HTML list elements, ensuring the hierarchy is clear and well-structured on the page.

Conclusion

Implementing an infinite category tree structure with PHP relies on efficient data recursion and presentation. This method is highly practical for use in content management systems, e-commerce sites, and anywhere multi-level category structures are needed. I hope this guide helps you better understand and apply the concept of infinite category tree structures in your projects.

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