If you click on a link and it opens in a new tab, in that new tab you have some Javascript that gets the referral url but the referral url equals nothing or undefined.
I want to know if it is possible to get the referral url even when the link has been opened in a new tab.
Yes this is doable. In the new tab's JavaScript you can write
alert(document.referrer);
Know more about Document Referrer.
You can also do this in PHP. Check this Get original URL referer with PHP