Well you can actually run the same script you know. A simple example on how to go around these things:
You create that index.php, and make it look like this, for example (assuming you've already done your PHP connection and there's a field 'article' and 'articlename' in a table 'tblContent':
$query = "SELECT article FROM tblContent WHERE articlename = 'apples'";
$query = "SELECT article FROM tblContent WHERE articlename = 'pears'";
$query = "SELECT article FROM tblContent WHERE articlename = 'main'";
$result = mysql_query($query) or die;
list($article) = mysql_fetch_row($result);
Now if you call your script like this: index.php?content=apples , the "apples" case will get executed, and the correct content will be loaded whereever you want it to be loaded (well, whereever you echo() it, in this case). Same thing for the other menu items of course. If none of the cases are true, the 'main' content will be loaded. And that's that. Have a lot of fun :-).