<?php

$arr = [
	'android' => [
		56 => '5.9',
		34 => '2.2'
	],
	'windows' => [
		23 => '7',
		56 => '8'
	]
];

function getTreeData($array, $values = []) {

    function walk_items(&$item, $key) {
        $item = ['text' => $item, 'value' => $key];
    }

    function walk_groups(&$item, $key) {
        $item = ['text' => $key, 'nodes' => array_values($item)];
    }

    array_walk_recursive($array, walk_items);
    array_walk($array, walk_groups);

    return json_encode(array_values($array));
}

var_export(getTreeData($arr));