<?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 = [];

foreach ($data->Chats as $Chat) {
    $ChatIds[] = $Chat->Chat->ChatId;
}

var_export($ChatIds);