If you’re not allowed to save the PHP files, you’ll need to allow the php extension. Go to System > System Settings and search for the upload_files setting. Edit it, and add php into that comma-separated list. After that, saving PHP files will work.
Do note that allow users to edit PHP files from the manager can lead to either wrecking the site (e.g. editing the root index.php or configuration file) or them gaining additional permissions. If you have other users you don’t 100% trust with manager access, it’s best to disable php editing again after you’re done.