nofishangler on "$_GET['lang'] returns "en" when it is supposed to return null"

When trying to get the "lang" string from a url that contains no "lang" string, "en" is returned.

For example
http://www.myadress.com/?lang=en
$_GET['lang'] will return "en", which is correct.

however when the url is
http://www.myadress.com/
$_GET['lang'] will also return "en" when it is supposed to return null.

Do you know what causes this?