PHP7.2中引入了新的数据类型object，在官方文档中说，object可以作为任何对象的参数、返回值的类型。如果当前的PHP环境低于7.2，那么PHP就会报
Return value must be an instance of SymfonyComponentDomCrawlerobject, instance of SymfonyComponentDomCrawlerCrawler returned