Issue
Your problem here is that you probably aren't using the correct permissions. To get cookies from a certain url/page you'll need to use the chrome.cookies API. This API needs the permissions for "cookies" and all the URLs you want to retrieve cookies from.
Solution
In your manifest.json, request the permissions you need. If you want to retrieve cookies from all the sites, you can use the "<all_urls>" permission. Or, if you want cookies only from that site you can use, for example, "http://luptaonline.com/*. I also noticed that you are using the chrome.tabs API, so the permissions in manifest.json should look like this:
...
"permissions": [
"cookies",
"tabs",
"<all_urls>"
],
...
Now, in your background.js, you can easily call the chrome.cookies.get() method to retrieve the PHPSESSID cookie, like this:
var myUrl = "http://luptaonline.com/player/"; // assuming that this is the url
chrome.cookies.get({url: myUrl, name: 'PHPSESSID'}, function(cookie) {
// do something with the cookie
console.log(cookie);
alert(cookie.name + ' found, value: ' + cookie.value);
});
Running the above code you'll get what you want. Here are some screenshot of the results:
Console log:

Cookie alert:

Working example
You can download a working example of the extension HERE.
Documentation
You may find this documentation links helpful: