Need help with conditional url?

I’m trying to make a simple conditional statement to add to my functions file.

I’m trying to redirect to one of two pages depending upon a string clicked in my url.
Something like this…

If_string(‘level_req=1’); // would redirect to page_id “234” and
if_string(‘level_req=0’); // would redirect to page id “345” or
all else would redirect to page id “456”