PHP - Allow Access to Page Only If Variable Exists in Url

I'm sorry I couldn't come up with a more fitting title but here is what hope to achieve with PHP:

• I have a page with url www.foo[.]com/mypage

• I only want that page or url to be accessible if it comes with ?user=$email so if someone tries to visit the url without ?user=$email, they get redirected somewhere else.

• How do i define that condition?