<?php

        $menuCategories = [
            120,
            120 => [1201 , 1202, 1203, 1204, 1205, 1206],
            121,
            121 => [1211, 1212, 1213],
            122,
            122 => [1221, 1222, 1223, 1224],
            123,
            123 => [1232],
            124,
            124 => [1241, 1242],
            125,
            125 => [1251, 1252],
            126,
            126 => [1261, 1262, 1263, 1264, 1265, 1266]
        ];
        $menuCategoriesFlat = [];
        array_walk_recursive($menuCategories, function($value) use (&$menuCategoriesFlat) { 
            $menuCategoriesFlat[] = $value;
        });
        var_dump($menuCategoriesFlat);