well.. i'm not a PHP coder (i'm a ColdFusion coder) but, the logic is the same...
well.. up there in the querystring (?something) you'll pass a value that will identify the page you wanna load....
so, you'll have to put some IFs (i recommend you to use SWITCH/CASE) and then include the page... like
(this is not the correct php syntax, just logical stuff)
SWITCH 'value':
CASE value = 'credits' {include credits.php};
CASE value = 'news' {include news.php};
or you may use IF/ELSE
IF (value = 'credits') then (include credits.php)
ELSE IF (value = 'news') then (include news.php)
ELSE IF (value = 'archive') then (include archive.php)
got the idea?