PHP natively doesn’t support automating tasks, you have to build a solution yourself or search google for available solutions. If you have a frequently visited site/page, you could add a timestamp to the database linking to the file, when visiting your site in a chosen time (e.g. 8 in the morning) the script (e.g. deleteOlderDocuments.php) runs and deletes the files that are older.
PHP operates under the request-response model, so it won’t be the responsibility of PHP to initiate and perform the scheduled job. Use cron, or make your PHP site to register the cron jobs.