Extract a complete branch from a Parent-Child data structure in PHP using recursion

Question

I have an object from a sql-query. Every entry has the keys (id, type, title, parent_id).

My example data:

Content of the sql-object $list (var_export()):

$array = array(0 => (object) array( 'id' => 1, 'type' => 'label', 'title' => 'Product Categories', 'parent_id' => 0, ), 
               1 => (object) array( 'id' => 2, 'type' => 'label', 'title' => 'Shoes', 'parent_id' => 1, ), 
               2 => (object) array( 'id' => 3, 'type' => 'label', 'title' => 'T-Shirts', 'parent_id' => 1, ), 
               3 => (object) array( 'id' => 4, 'type' => 'label', 'title' => 'With Print', 'parent_id' => 2, ), 
               4 => (object) array( 'id' => 5, 'type' => 'label', 'title' => 'Without Print', 'parent_id' => 2, ), 
               5 => (object) array( 'id' => 6, 'type' => 'label', 'title' => 'Brands', 'parent_id' => 2, ), 
               6 => (object) array( 'id' => 7, 'type' => 'label', 'title' => 'Blue', 'parent_id' => 3, ), 
               7 => (object) array( 'id' => 8, 'type' => 'label', 'title' => 'Red', 'parent_id' => 3, ));

What i expect:

the function, should find the dependencies of the entries starting with a given id. Here for example the ID 7:

Array
(
    [0] => stdClass Object
        (
            [id] => 7
            [type] => "label"
            [title] => "Blue"
            [parent_id] => 3
        )
    [1] => stdClass Object
        (
            [id] => 3
            [type] => "label"
            [title] => "T-Shirts"
            [parent_id] => 1
        )
    [2] => stdClass Object
        (
            [id] => 1
            [type] => "label"
            [title] => "Product Categories"
            [parent_id] => 0
        )
)

What i get:

I just get an array with the first entry, with the id I started with. As example with starting ID 7:

array ( 0 => (object) array( 'id' => 7, 'type' => 'label', 'title' => 'Blue', 'parent_id' => 3, ), ) 

My current Function:

The function needs to search for the item with the given id, stores the information into a new array and then start a new search but with the parent_id as new search id. This should loop as long as there are dependencies, if there are no dependencies the loop should stop and returning the created array.

function getParentSelect($list, $parent) {
    $next_id = true;
    $result = array();    
    foreach($list as $k => $s) {
        echo $s->id;
        if ($s->id == $parent) {
            $result[] = $s;
            $next_id = $s->parent_id;                                     
            break;
        }
        else {
            $next_id = false;
        }
    }
    if ($next_id != false) {
        $result = array_merge($result, getParentSelect($list, $next_id));
    }
    return $result;
}

Answer 1

Recursion is always somewhat difficult to understand. I think you got the main idea, but the execution was flawed. This is what I can up with:

function getParentSelect($list, $select_id) {
    $result = [];    
    foreach($list as $s) {
        if ($s->id == $select_id) {
            $result = array_merge([$s], getParentSelect($list, $s->parent_id));
        }
    }
    return $result;
}

The assumption here is that all the parent id's are valid.

How does the code work?

The function itself searched the whole list for items with the id that was selected. If it finds one it will add it to the results, but it also looks for any parents of that item. This is where the function recurses. This means that the function can also look for parents of parents, and so on. array_merge() is used to combine the items and all parents together to form the results.