Online PHP editor | refs for RAO45

<?php $links = [ [ 1, 3], [ 3, 5], [ 3, 7], [ 3, 9], [ 1, 10], [10, 15], ]; $input = array_map(function ($link) { return ['id' => $link[1], 'parent' => $link[0]]; }, $links); function buildTreeMap($input, $child = 'id', $parent = 'parent') { $map = array(); foreach ($input as $node) { // init self if (!array_key_exists($node[$child], $map)) { $map[$node[$child]] = []; } // init parent if (!array_key_exists($node[$parent], $map)) { $map[$node[$parent]] = []; } // add to parent $map[$node[$parent]][$node[$child]] =& $map[$node[$child]]; } return $map; } $map = buildTreeMap($input); // the whole map print_r($map); // only from root 1 print_r($map[1]);