Nested arrays in parser with conditional rendering

Code:

CREATE TABLE `categories` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `parent_id` int(11) NOT NULL DEFAULT '0',
  `category` varchar(255) NOT NULL,
  PRIMARY KEY (`id`)
)

-----------------------------------------------------------

INSERT INTO `categories` (`id`, `parent_id`, `category`) VALUES
(1, 0, 'General'),
(2, 0, 'PHP'),
(3, 0, 'HTML'),
(4, 3, 'Tables'),
(5, 2, 'Functions'),
(6, 2, 'Variables'),
(7, 3, 'Forms');

PHP Code:

public function generateTree(array $data, int $parent = 0, int $depth = 0): string
{
    $tree = "<ul>\n";
    for ($i=0, $ni=count($data); $i < $ni; $i++) {
        if ($data[$i]['parent_id'] == $parent) {    
            
            $tree .= "<li>\n";
            $tree .= $data[$i]['category'];
            $tree .= generateTree($data, $data[$i]['id'], $depth+1);
            $tree .= "</li>\n";
        }
    }

    $tree .= "</ul>\n";

    return $tree;
} 


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