My permalink is set to custom structure:/%postname%.php in WordPress. It is fine for posts. But the permalink for pages does not end with .php. If I change the permalink when editing a page, i.e., change "my-page" to "my-page.php", after I click "ok", the link changes to "my-page-php" rather than "my-page.php". How to add a suffix to the permalink of pages?