You'll want to have your include for the content inside the body of your html, however, this isn't the only problem!
Take a look at this:
content.php
$id = $_GET['id'];
switch($id) {
case 1:
include 'hello.html';
break;
case 2:
include 'blank.html';
break;
default:
include 'default.html';
break;
}
A case structure (defined by the switch() function) is basically an if statement. What it's saying is that,
if the id is equal to 1, then include hello.html, then break out of the case, so it doesn't keep checking. if it's NOT equal to 1, then check if it's equal to 2, and so on.
For your links page, you'll want this:
<html>
<head>
<title>Testing Links</title>
</head>
<body>
<a href="content.php?id=1>Happy House</a>
<?php
include("content.php");
?>
</body>
</html>
You put it there so that the content is placed inside the body of your document, otherwise it would be placed up the top. This isn't a great thing for valid html 🙂
Hope that helps,
Matt