Get categories and subcategories in one query, seen many solutions that get the categories and for each category check for subcategories. Uses codeigniter framework, fairly easy to convert to vanilla php if you need. Probably a better storage/display solution than the strange array that I used.

controller.php

<?php
$cats_result = $this->model->get_categories($swapid);
if ($cats_result->num_rows() > 0) {	
	$cat_index = $sub_index = $row_index = 0;
	
	do {
		$cats[$cat_index]['cat'] = array(
			'name' => $cats_result->row($row_index)->cat_name,
			'title' => $cats_result->row($row_index)->cat_title
		);
	
		if($cats_result->row($row_index)->sub_name !== null){
			do {
				$cats[$cat_index]['subs'][$sub_index] = array(
					'name' => $cats_result->row($row_index)->sub_name,
					'title' => $cats_result->row($row_index)->sub_title
				);
				
				$sub_index++;
				$row_index++;
	
				if ($cats_result->row($row_index)->cat_name === $cats_result->row($row_index - 1)->cat_name)
					continue;
				break;
	
				if ($row_index > 50) break 2;
	
			} while (true);
		} else {
			$row_index++;
		}
	
		$sub_index = 0;
		$cat_index++;
	} while ($row_index <= $cats_result->num_rows() + 1);
}

model.php

<?php
public function get_categories($swap)
{
	$sql = 'select 
			    `categories`.`title` AS `cat_title`,
			    `categories`.`name` AS `cat_name`,
			    `subcategories`.`title` AS `sub_title`,
			    `subcategories`.`name` AS `sub_name`
			from
			    `subcategories`
			        join
			    `categories` ON `categories`.`ID` = `subcategories`.`parent_id`
			        join
			    `swaps_looking_for_subcat` ON `swaps_looking_for_subcat`.`category_id` = `subcategories`.`ID`
			where
			    `swaps_looking_for_subcat`.`swap_id` = ' . $swap . '
			UNION select 
			    `categories`.`title` AS `cat_title`,
			    `categories`.`name` AS `cat_name`,
			    `subcategories`.`title` AS `sub_title`,
			    `subcategories`.`name` AS `sub_name`
			from
			    `categories`
			        left outer join
			    `subcategories` ON `subcategories`.`parent_id` = `categories`.`ID`
			        join
			    `swaps_looking_for_cat` ON `swaps_looking_for_cat`.`category_id` = `categories`.`ID`
			where
			    `swaps_looking_for_cat`.`swap_id` = ' . $swap . '
			order by
				`cat_title`, `sub_title`';
	
	return $this->db->query($sql);
}

view.php

<?php
if ($cats) {
	// echo '<pre>';
	// print_r($looking_for_cats);
	// echo '</pre>';
	echo '<ul>';
	foreach ($cats as $cat) {
		echo '<li><a href="/browse/' . $cat['cat']['name'] . '">' .  $cat['cat']['title'] . '</a></li>';
		if (isset($cat['subs'])){
			echo '<ul>';
			foreach ($cat['subs'] as $sub) {
				echo '<li><a href="/browse/' . $cat['cat']['name'] . '/' . $sub['name'] . '">' .  $sub['title'] . '</a></li>';
			}
			echo '</ul>';
		}
	}
	echo '</ul>';
}