How to prevent HTTP_REFERER from being overwritten in Wordpress?

Question

In the header.php file of my WordPress site I have the following code:

<?php
session_start();  
$_SESSION['refererurl'] = $_SERVER['HTTP_REFERER'];
?>

To test the above code, I placed the following code in the front-page.php file (and it works):

<?php
    echo $_SESSION['refererurl'];
?>

I want to track the referrer URL that led a visitor to my site from an external site for tracking purposes. The problem is that $_SERVER['HTTP_REFERER'] resets on page-load, so while the information is useful when a visitor first lands on my website, it's overwritten whenever the visitor navigates further into my site.

Is there any way to store the value of $_SERVER['HTTP_REFERER'] when a visitor first arrives at my site from an external site for as long as the visitor is on my site?

Answer 1

Very simple - only set it if it isn't already set.

if(!isset($_SESSION['refererurl'])) {
$_SESSION['refererurl'] = $_SERVER['HTTP_REFERER'];
}