<?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]],
  ]
];

$Chats = array_column($data->Chats, "Chat");
$ChatIds = array_column($Chats, "ChatId");

var_export($ChatIds);