How To Know Referer Link Of A Visitors In Your Website In PHP

 <?php
  if(isset($_SERVER['HTTP_REFERER'])) {
      echo $_SERVER['HTTP_REFERER'];
   }
   ?>

This code placed on that page which is user want to visit 
When a visitor visits this page it saves his referrer link in  $_SERVER['HTTP_REFERER']; this variable now insert this value to your database