@Howdy_McGee not quite. it is common, but if you have classes that will only be ever used together then they might make sense in one file. one example I remember some Symfony components do that for really small classes. the more important practice is to keep definitions apart from any logic.
@MarkKaplun autoload can handle multiple classes in a file just fine (file just get loaded whenever any of them is requested), also file name it irrelevant to many autoload implementations (such as Composer's map mode).