The referer - which is actually a misspelling for the word referrer - is a commonly used and powerful tool. It points out on which website the link was located that the current visitor clicked to visit your website. Both web designers and SEOs can profit greatly from knowing where a visitor came from. This is the case for both internal clicks on a website, and clicks from an external website (e.g., Google) to your website. But how is this done? And when should or shouldn't you use it?

Using the HTTP referer is actually a simple task. First, check if the referer is set and whether the referer is not empty. If the referer is both set and not empty, then start processing it. In this case we will output the referer by using the language construct 'echo' protected by the function htmlspecialchars().

if( isset($_SERVER['HTTP_REFERER']) AND trim($_SERVER['HTTP_REFERER']) != '' )

{

 # Start processing the referer here

 echo 'Referer: '. htmlspecialchars($_SERVER['HTTP_REFERER']);

}

Using the referer as a safety measure

Some people seem to have figured, "Since it is possible to see what page my visitors came from, I can use this as a way to grant specific people access to my SuperSecretSite." Thus, they grant access to certain domains on their white list, or block access to domains on their black list. This is bad thinking. Since faking the HTTP_REFERER is a fairly simple task using telnet or XmlHttpRequest, SuperSecretSite will have a major security breach. In short, never ever use HTTP_REFERER as a security measure.

Catering for specific visitors

If you have a partnership with another website, say friendlysite.com, you probably have your link displayed on that page. Sometimes it can be useful to let a visitor know you are affiliated to friendlysite.com or offer a special discount for visitors from friendlysite. This can be done as follows:

if( isset($_SERVER['HTTP_REFERER']) AND trim($_SERVER['HTTP_REFERER']) != '' )

{

 # Split the referer into pieces

 aUrlParts = parse_url($_SERVER['HTTP_REFERER']);

 # Check whether the host check contains the phrase 'friendlysite.com'

 if( strpos($urlParts['host'], 'friendlysite.com') !== false )

 {

  echo 'Special price for special friend from friendlysite.com!';

 }

}

Sending a visitor back from where he came from

When a user wishes to log out on a website, they don't want to be send to a blank page displaying 'You have successfully logged out!'. No, they want to remain on the same webpage, and just have their account signed off. Referer to the rescue!

# First perform all the logout logic

if( isset($_SERVER['HTTP_REFERER']) AND trim($_SERVER['HTTP_REFERER']) != '' )

{ 

 header('Location: '. $_SERVER['HTTP_REFERER']);

}

else

{

 # Referer is not set, so pass on that visitor to our mainvpage

 header('Location: http://www.ourownwebsite.com/');

}

Tracking a user's path 

We all want to know where our visitors came from. It tells us which links are useful, and which are not. It helps you track the effectiveness of campaigns and shows you on which sites your recent article is being linked to. But how can you do this?

if( isset($_SERVER['HTTP_REFERER']) AND trim($_SERVER['HTTP_REFERER']) != '' )

{

 # Insert the referer into your favourite database here

}