php如何判断手机系统

header("Content-type:text/html; charset=utf-8");
$user_agent = $_SERVER['HTTP_USER_AGENT'];//返回手机系统、型号信息
//var_dump($user_agent);
if(stristr($_SERVER['HTTP_USER_AGENT'],'Android')){//返回值中是否有Android这个关键字
   //echo'你的手机是：Android系统';
header('Location: http://xxx');
}else if(stristr($_SERVER['HTTP_USER_AGENT'],'iPhone')){
   //echo'你的手机是：IOS系统';
header('Location: https://xxx');
}else if(stristr($_SERVER['HTTP_USER_AGENT'],'WindowsPhone')){
   //echo '你使用的是WindowsPhone系统';
header('Location: http://xxx');
}else{
header('Location: http://xxx');
}