Category Hierarchy in PHP/MySQL
When dealing with hierarchical data, such as categories, it's crucial to represent them in a structured manner. One common approach is using an adjacency list model, where each row represents a category and has a field indicating its parent category.
Obtaining the Hierarchy
Using PHP, you can retrieve the category data from MySQL and build the hierarchy in a single pass. The code below, inspired by Nate Weiner's "One Pass Parent-Child Array Structure":
<code class="php">$refs = []; $list = []; $sql = "SELECT category_id, parent_id, category_name FROM categories ORDER BY category_name"; foreach ($result as $row) { $ref = &$refs[$row['category_id']]; $ref['parent_id'] = $row['parent_id']; $ref['category_name'] = $row['category_name']; if ($row['parent_id'] == 0) { $list[$row['category_id']] = &$ref; } else { $refs[$row['parent_id']]['children'][$row['category_id']] = &$ref; } }</code>
This loop populates two arrays: $refs contains references to each category, while $list contains the top-level categories (with no parents).
Creating a Hierarchical Representation
To create a visual representation of the hierarchy, you can use a recursive function to generate a nested HTML list. Consider the following code:
<code class="php">function toUL(array $array) { $html = '<ul>' . PHP_EOL; foreach ($array as $value) { $html .= '<li>' . $value['category_name']; if (!empty($value['children'])) { $html .= toUL($value['children']); } $html .= '</li>' . PHP_EOL; } $html .= '</ul>' . PHP_EOL; return $html; }</code>
This function takes an array of categories as input and recursively builds a nested HTML list. The resulting HTML can then be outputted to display the category hierarchy.
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