Warum muss das denn so schwer sein?
Und dabei mit if arbeiten? Viel Spaß bei 25 Menüpunkten. 
Workarond ...
PHP-Code:
<?php
switch(intval($_GET['id']))
{
case 1: $pageCall = 'page1'; break;
case 2: $pageCall = 'page2'; break;
case 3: $pageCall = 'page3'; break;
default: $pageCall = 'page1';
}
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style type?"text/css">
*{
border: 0;
padding: 0;
margin: 0;
}
html{
background: #fff;
color: #000;
font-size: 1em;
font-family: Arial, Helvetica, Verdana, sans-serif;
}
#navi li{
float: left;
list-style: none;
margin-top: 0;
}
#navi li a{
display: block;
}
#navi li#n1 a,
#navi li#n2 a,
#navi li#n3 a{
background: #fff;
}
body#page1 #navi li#n1 a,
#navi li#n1 a:hover,
#navi li#n1 a:active{
background: #9ff;
}
body#page2 #navi li#n2 a,
#navi li#n2 a:hover,
#navi li#n2 a:active{
background: #9f9;
}
body#page3 #navi li#n3 a,
#navi li#n3 a:hover,
#navi li#n3 a:active{
background: #f99;
}
.cleaner{
clear: both;
float: none;
}
</style>
</head>
<body id="<?php echo $pageCall; ?>">
<ul id="navi">
<li id="n1"><a href=<?php echo $_SERVER['PHP_SELF']; ?>?id=1>page1</a></li>
<li id="n2"><a href=<?php echo $_SERVER['PHP_SELF']; ?>?id=2>page2</a></li>
<li id="n3"><a href=<?php echo $_SERVER['PHP_SELF']; ?>?id=3>page3</a></li>
</ul>
<br class="cleaner" />
<div id="content">
Hier dann ein include(<?php echo $pageCall; ?>.inc.php)
</div>
</body>
</html>