This may be a very simple question but I don't what's the wrong thing am doing here...

To explain you clearly, I've set a real simple example below:

<ul>
    <li><a href="test.php?link1=true">Link 1</a></li>
    <li><a href="test.php?link2=true">Link 2</a></li>
    <li><a href="test.php?link3=true">Link 3</a></li>
</ul>

<?php 
    if(isset($_GET['link1'])) {
        if(($_GET['link1']) == 'true') {
            echo 'This Is Link 1';
        }
    }

    if(isset($_GET['link2'])) {
        if(($_GET['link2']) == 'true') {
            echo 'This Is Link 2';
        }
    }

    if(isset($_GET['link3'])) {
        if(($_GET['link3']) == 'true') {
            echo 'This Is Link 3';
        }
    }
?>

This is a test.php page, here I've set 3 different arguments for $_GET, and show contents accordingly, now everything works perfect, the only thing am not understanding is how to block this kind of url say

if user clicks on link 1 the url will be :

http://localhost/test.php?link1=true

And the Output of this url is This is Link 1

Now if I change this url to :

http://localhost/test.php?link3=true&link2=true&link1=true

And the Output what I get is This Is Link 1This Is Link 2This Is Link 3

Now this is ok here, but it's very annoying if someone types this and see's forms one below the other, any way I can stop this tampering?