You could do something like this:
<html>
<head>
<title>My Website</title>
</head>
<body>
<table width="100%" border="0" cellpadding="2" cellspacing="3">
<tr>
<td id="sidebar">
<a href="?go=home">Home</a><br /><br />
<a href="?go=page-1">Page 1</a><br />
<a href="?go=page-2">Page 2</a><br />
<a href="?go=page-3">Page 3</a><br />
</td>
<td id="content">
<?php
$default = "content/home.php";
if (isset($_GET['go'])) {
switch ($_GET['go']) {
case "page-1": include("content/page1.php"); break;
case "page-2": include("content/page2.php"); break;
case "page-3": include("content/page3.php"); break;
default: include($default);
}
} else { include($default); }
?>
</td>
</tr>
</table>
</body>
</html>