Suppose my site's url is given as hyperlink on some page on the internet; that page could be anything on internet - blog, orkut, yahoo, even stackoverflow etc, and someone clicks on it,and visited my site. So can we know, using php, the previous url from which the visitor came to my page?
Use the $_SERVER['HTTP_REFERER'] header, but bear in mind anybody can spoof it at anytime regardless of whether they clicked on a link.
I can't add a comment yet, so I wanted to share that HTTP_REFERER is not always sent.
$_SERVER['HTTP_REFERER'] will give you incomplete url.
If you want http://bawse.3owl.com/jayz__magna_carta_holy_grail.php, $_SERVER['HTTP_REFERER'] will give you http://bawse.3owl.com/ only.
But you could make an own link for every from url.
Example: http://example.com?auth=holasite
In this example your site is: example.com
If somebody open that link it's give you the holasite value for the auth variable.
Then just $_GET['auth'] and you have the variable. But you should have a database to store it, and to authorize.
Like: $holasite = http://holasite.com (You could use mysql too..)
And just match it, and you have the url.
This method is a little bit more complicated, but it works. This method is good for a referral system authentication. But where is the site name, you should write an id, and works with that id.
$_SERVER['HTTP_REFERER'] is the answer
Source: Stackoverflow.com