Validating PHP URL variables

Hello,
Hopefully this is a simple problem.

I have php page using includes that load content via url variables similar to

http://www.mywebsite.com/main.php?page=services&title=servicesTitle

Right now if someone manually changes the variables in the url to say
http://localhost/main.php?page=services&title=fake
it still loads the page minus the include content because the variable is wrong.

How do I check to make sure that the variable contents are correct and if not send them to an error page?

Thanks!