简单的写法，直接or判断域名
 <?php 
if (($_SERVER['HTTP_HOST']!='www.xx.com') || ($_SERVER['HTTP_HOST']!='1.xx.com') || ($_SERVER['HTTP_HOST']!='1.xx.com'))
{ 
echo (" <script type='text/javascript'> alert('禁止访问!');history.go(-1); </script>");
} 
?>

方便的写法，pre_match匹配字符，最好再加一些其他限定，具体参考正则

 <?php 
if ( !preg_match("/xx.com/i",$_SERVER['HTTP_HOST'])) //如果域名中不包含xx.com
{ 
echo (" <script type='text/javascript'> alert('禁止访问!');history.go(-1); </script>"); 
} 
?>