I have a lang system in my website. When people click to switch to another language, the following will be added to the url: ?lang=. The problem is, my website is divided by pages which add ?p= to the url aswell. So basically, if they change the lang on one of those pages, it'll overwrite the ?p= and go back to the main page. So it'll be index.php?lang=.
What is the code or how should I code it so that php verifies if there's already a ? string in the url and switch ?lang= to &lang=?