使用isset显示页面内容

I am having an issue using isset to display content on a page.

My PHP file is called messages.php

I am directing my users with links to this URL: messages.php?inbox using if(isset($_GET['inbox'])) { } to display the users inbox. Same principle with the other users options such as compose message is: messages.php?compose again using isset

The only problem I have is that I cannot stop people from manually typing stuff like domain.com/messages.php or domain.com/messages.php?somethingrandom.

Is there a way to direct users to messages.php?inbox when they type in the address bar something that isnt assigned to isset?

I did try to use switch but couldnt seem to get it to work properly with how ive laid out my HTML.

An example of the whole file is here http://pastebin.com/SfqN2L7g

I am fairly new to PHP and think I may have gone down the complicated route.

Any advice would be appreciated.

Thanks

The answer you added already would work, but I usually like having an array of valid options which I could maybe check against later on.

$validPages = array('inbox', 'compose');
$pageFound = false;

foreach ($validPages as $validPage) {
    if (isset($_GET[$validPage])) {
        $pageFound = true;
        break;
    }
}

if (! $pageFound) {
    header('Location: /messages.php?inbox');
}

Thanks to the help of Marcos Pérez Gude, the answer is as follows:

if(isset($_GET['inbox']) || isset($_GET['compose'])){

//Then do below

 }else{
    header("Location: messages.php?inbox");
    exit;
}