question-1.php

<?php

$a = [
2 => 1,
3 => 1,
4 => 1,
6 => 5,
7 => 5,
8 => 5,
9 => 7,
10 => 7,
11 => 10,
];

function trans($a) {
	$pairs = [];
	foreach ($a as $k => $v) {
		if(isset($pairs[$v])) {
			$pairs[$v] []= $k;
		}
		else {
			$pairs[$v] = [$k];
		}
	}

	$root_ids = array_diff(array_unique(array_values($a)), array_keys($a));
	return build($root_ids, $pairs);
}

function build(&$ids, &$pairs) {
	return array_map(function($id) use ($pairs) {
		$item = ['id' => $id];
		if(isset($pairs[$id])) {
			$item['items'] = build($pairs[$id], $pairs);
		}
		return $item;
	}, $ids);
}

$b = trans($a);
print_r($b);
echo json_encode($b, JSON_PRETTY_PRINT) . "\n";