First step
Declare all your functions in your case.php file i.e
switch($op) {
# page1
case "function1":
# page2
case "function2":
# page3
case "function3":
# page4
case "function4":
include("modules/$module_name/admin/index.php");
break;
In you links file declare the first function to call from your admin panel
i.e.
adminmenu("".$admin_file.".php?op=function1", "Yourmodule", yourmodule.gif");
For jumping between files, do links like this
You're in file A and you want to call function functionzzzzz in your file B do
<a href=\"".$admin_file.".php?op=functionzzzzz\">"._o k."</a>";