... you need to write it.
But instead just use a switch. Create a function for each page you'll want. page_1(), page_2(), etc... then in the righthand cell include this:
switch($_GET['id']) {
case 1:
page_1();
break;
case 2:
page_2();
break;
default:
page_default();
break;
}
The break statements are necessary, the default case should come last, each of those page functions will display whatever you'd like them too, the $_GET['id'] comes from the ?id=# following the URL in the link.
If you'd like to use string identifiers instead of numbers, they need to be in quotes in the switch:
case "contacts":
page_contacts();
break;