Reference parent child matrix

function test_reference_tree_building() {
        $source = [
            1 => ['name' => 'Parent One', 'parent_id' => null], 2 => ['name' => 'Parent Two', 'parent_id' => null],
            3 => ['name' => 'Child One', 'parent_id' => 1], 4 => ['name' => 'Child Two', 'parent_id' => 1],
            5 => ['name' => 'Child Three', 'parent_id' => 2], 6 => ['name' => 'Child Four', 'parent_id' => 5],
            7 => ['name' => 'Child Five', 'parent_id' => 2], 8 => ['name' => 'Child Six', 'parent_id' => 3],
            9 => ['name' => 'Child Seven', 'parent_id' => 6]
        ];
        $nested = [];
        foreach ( $source as &$s ) {
            if ( is_null( $s['parent_id'] ) ) {
                $nested[] = &$s;
            } else {
                $pid = $s['parent_id'];
                if ( isset( $source[$pid] ) ) {
                    // If the parent ID exists in the source array
                    // we add it to the 'children' array of the parent after initializing it.
                    if ( !isset( $source[$pid]['children'] ) ) {
                        $source[$pid]['children'] = array();
                    }
                    $source[$pid]['children'][] = &$s;
                }
            }
        }
    }