You have to check for the existence of the $_GET var before you can use it.
And, as others have said, always turn on reporting while your coding (then turn it off in production!), PHP will always tell you what your doing wrong when there's a problem.
Also, there's no need to do a long switch statement for what you're trying to accomplish. If the file names are the same as the 'page' key values, then all you need to do is run a file_exists() check to validate the value as a real file, or return an error if the file doesn't exist.
You could even write a function to read the contents of the 'includes' directory and generate the menu list dynamically. This way, in order to add a page to your site, you would only need to drop a new file into the directory and it would automatically show up in your menu.
Also, as an aside, you should always use unordered lists for menus. Much easier to style & control the display...
Here's one simple example of how to better write your script... It works:
<?php
error_reporting(E_ALL);
ini_set('display_errors', 'on');
?>
<html>
<head></head>
<body>
<!-- Below is an unordered list for easier styling.
Also note that there's no need to have 'index.php' in your links if it's your default file.
Additionally, the home link doesn't need it's own GET var, we just set it as the default value below. -->
<ul class="menulinks">
<li><a href="/">Home</a></li>
<li><a href="?page=about">Über mich</a></li>
<li><a href="?page=services">Leistungsspektrum</a></li>
<li><a href="?page=astro">Astrologie</a></li>
<li><a href="?page=relax">Entspannungspädagogik</a></li>
<li><a href="?page=family">Familienstellen</a></li>
<li><a href="?page=contact">Kontakt</a></li>
<li><a href="?page=route">Anfahrt</a></li>
</ul>
<div id="content">
<?php
// if the GET var isn't set, we default the file name to the home file
if(!isset($_GET['page'])) {
$file = 'home.php';
// otherwise, we set the filename to the GET var value
} else {
$file = $_GET['page'] . '.php';
}
// Here we set the absolute path to the file so we can check if it exists
$filepath = dirname(__FILE__) . '/includes/' . $file;
// if the file doesn't exist, echo an error message
if(!file_exists($filepath)) {
echo "<p>The file $file does not exist!</p>";
// otherwise, include the file & we're done!
} else {
include($filepath);
}
?>
</div>
</body>
</html>
