Category Hierarchy (PHP/MySQL)

I am trying to get my all categories and sub-categories from MySQL database in a hierarchy:

My result should be like that (just example):

Cat A

Sub-Cat 1

Sub_Sub_Cat 1

Sub_Sub_Cat 2

Sub_Cat 2

Cat B

Cat C

...

MySQL code:

CREATE TABLE IF NOT EXISTS `categories` (
   `category_id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
   `parent_id` mediumint(8) unsigned NOT NULL DEFAULT '0' COMMENT 'for sub-categories'
  PRIMARY KEY (`category_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 ;

Simply, how can get it in a hirarchy with PHP codes?

Solution

When using an adjacency list model, you can generate the structure in one pass.

Taken from One Pass Parent-Child Array Structure (Sep 2007; by Nate Weiner):

$refs = array();
$list = array();

$sql = "SELECT item_id, parent_id, name FROM items ORDER BY name";

/** @var $pdo \PDO */
$result = $pdo->query($sql);

foreach ($result as $row)
{
    $ref = & $refs[$row['item_id']];

    $ref['parent_id'] = $row['parent_id'];
    $ref['name']      = $row['name'];

    if ($row['parent_id'] == 0)
    {
        $list[$row['item_id']] = & $ref;
    }
    else
    {
        $refs[$row['parent_id']]['children'][$row['item_id']] = & $ref;
    }
}

From the linked article, here's a snippet to create a list for output. It is recursive, if there a children for a node, it calls itself again to build up the subtree.

function toUL(array $array)
{
    $html = '<ul>' . PHP_EOL;

    foreach ($array as $value)
    {
        $html .= '<li>' . $value['name'];
        if (!empty($value['children']))
        {
            $html .= toUL($value['children']);
        }
        $html .= '</li>' . PHP_EOL;
    }

    $html .= '</ul>' . PHP_EOL;

    return $html;
}