Question: I would like to do something like play.clubpenguin.com. 

What it does is that, when you visit the site, maybe using firefox or IE, if you opened a new tab or use other browser, when you visit that site again, it will show something like: "Please close the other browser that opened this page" or something like that. 

How to do this? (Its client's request)

More information: The site is Flash site

EDIT: 

Ok, i think this is a tough question, well, in general, Can this be done using php, mysql and JS?

Answer: 
try using the below code:

```
    window.onload = function(){
    if (document.cookie.indexOf("_instance=true") === -1) {
    document.cookie = "_instance=true";
    // Set the onunload function
    window.onunload = function(){
        document.cookie ="_instance=true;expires=Thu, 01-Jan-1970 00:00:01 GMT";
    };
    // Load the application
    }
    else {
    // Notify the user
    }
    };

```

The code will restrict the user to open one browser tab at a time and during browser refresh(current tab) the code will not show any alert. Copy pasting the same URL in new tab will not be allowed the user to open. for more info try [this](http://kinsey.no/blog/index.php/2009/11/15/how-to-restrict-your-web-app-to-a-single-instance/)
Solution:

Response your clients request with a NO, because you are the webdesign guru that knows what's the best for the client's visitors. the website doens't have to appeal to your client but the client's visitors. and when they are limited so hard on their own computers they are everything but satisfied.