Change GET's values

Change GET's values

I want to change my GET's values in PHP.
For example, I want to change this:
mypage.com/?lang=fr&page=home&...
To this:
mypage.com/?lang=en&page=home&...
I could make it with an str_replace and substr but I'm looking a less
stupid way.
I would like to use $_SERVER['QUERY_STRING'].
Thanks.