<?php

$data = (object) [
  "Chats" => [
    (object) ["Chat" => (object) ["ChatId" => 1000]],
    (object) ["Chat" => (object) ["ChatId" => 2000]],
    (object) ["Chat" => (object) ["ChatId" => 3000]],
    (object) ["Chat" => (object) ["ChatId" => 4000]],
    (object) ["Chat" => (object) ["ChatId" => 5000]],
  ]
];

$ChatIds = array_map(function ($chat) {
  return $chat->Chat->ChatId;
}, $data->Chats);

var_export($ChatIds);