How to redirect users based on browser language

1. I want to redirect u开发者_JAVA百科sers based on browser language, I figured out how to do this here, it works fine, here is the code(PHP):

   if(preg_match('/en-US/', $_SERVER['HTTP_USER_AGENT']))
       header("location:index.php");
   else
       header("location:http://cn.gearor.com");
   

2. The problem is I only want to redirect users from other websites or at the first visit to my website. Which means I don't want users reading some pages of my website, when they go back to index, they were redirect to other page. And I also have a link in my website, like: English or Chinese, when users click English from a Chinese page, they will go to index.php and been redirect back to Chinese page, it makes users can't visit the English page. How do I fix this in PHP or JavaScript?

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You can set a session variable like $_SESSION['lang'] and only apply the above code if $_SESSION['lang'] is not set. what I mean is:

if(!isset($_SESSION['lang'])){
  if(preg_match('/en-US/', $_SERVER['HTTP_USER_AGENT'])){
    $_SESSION['lang'] = 'en';
    header("location:index.php");
  }else{
    $_SESSION['lang'] = 'other';
    header("location:http://cn.gearor.com");
  }
}

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Well it was hard to understant what you are saying, if you can reclarifiy a bit, but here is what I think you were asking for, Pastebin.