How can recursion be used to generate a nested menu tree from a database with parent categories?

Recursion for Menu Tree Generation

In your situation, you have a database structure where categories have a 'root' field indicating their parent category. The HTML output you desire involves nested lists representing the category hierarchy. To achieve this, a recursive PHP function can be employed.

Here's an example function:

<code class="php">function recurse($categories, $parent = null, $level = 0) {
    $ret = '<ul>';
    foreach ($categories as $index => $category) {
        if ($category['root'] == $parent) {
            $ret .= '<li>
<a href="#"><p class="Tier' . $level . '">' . $category['name'] . '</p></a>';
            $ret .= $this->recurse($categories, $category['id'], $level + 1);
            $ret .= '</li>';
        }
    }
    return $ret . '</ul>';
}</code>

This function assumes that root categories have a 'root' field with a null value. It iterates through the categories, identifies child categories based on their 'root' field, and constructs the HTML using nested lists.

To use this function:

1. Query your database for a multi-dimensional array of categories.
2. Call the recurse function with the array of categories and null as the parent parameter.
3. Assign the returned value to a variable (e.g., $Tree).
4. Echo the $Tree variable to display the nested menu tree.

Optionally, you can modify the function to avoid empty lists by checking if the category has child categories before creating the list:

<code class="php">function recurse($categories, $parent = null, $level = 0) {
    $ret = '<ul>';
    foreach ($categories as $index => $category) {
        if ($category['root'] == $parent) {
            $ret .= '<li>
<a href="#"><p class="Tier' . $level . '">' . $category['name'] . '</p></a>';
            $sub = $this->recurse($categories, $category['id'], $level + 1);
            if ($sub != '<ul></ul>') {
                $ret .= $sub;
            }
            $ret .= '</li>';
        }
    }
    return $ret . '</ul>';
}</code>

This modified function will only create lists if the category has at least one child.

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