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We want to add a new element to the $cpa array in each call to the function. To do this we need always to add to the original empty array. The ampersand allows us to to this. Without it, a copy of the array would be passed to the function and we would just keep adding to a new empty array each time. &$cpa passes the array by reference (ie its address in memory) instead of a copy. P.S. This method below (which stores all the category data into a $cat_data array instead of running a query in every call to the function, is 5x faster. $cat_data looks like this... Array ( [532] => Array ( [name] => Motorbikes::1 [parent] => 0 ) [533] => Array ( [name] => Cars::2 [parent] => 0 ) [534] => Array ( [name] => Boats::3 [parent] => 0 ) [535] => Array ( [name] => Bicycles::4 [parent] => 0 ) . . . ) CODE $cat_data = []; $res = $pdo->query("SELECT id , CONCAT(name, '::', position) as name , parent FROM category "); foreach ($res as $r) { $cat_data[$r['id']] = [ 'name' => $r['name'], 'parent' => $r['parent'] ] ; } $category_path_array = []; retrieve_category_path ($cat_data, 552, $category_path_array); $breadcrumbs = join('/', $category_path_array); echo $breadcrumbs; function retrieve_category_path (&$cats, $id, &$cpa) { array_unshift($cpa,$cats[$id]['name']); if ($cats[$id]['parent']) { retrieve_category_path($cats, $cats[$id]['parent'], $cpa); } } Thank you - much appreciated.