non si capisce bene la struttura dei file

aggiungendo semplicemente body dopo il tuo codice, funziona correttamente

codice:
<?php
function html_heading($title)
	{	
	$site_css=$_GET["css"]; 
	if(!$site_css || !strstr($site_css,".css"))
		$css="default.css";
	else 
		$css=$site_css;
		
?><!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>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title><?php echo($title); ?></title>

<link href="css/sites/<?php echo($css);?>" rel="stylesheet" type="text/css" />
<link href="css/style.css" rel="stylesheet" type="text/css" />
</head>

<?php }?>



<body>
	

	

</body>
</html>