Member Avatar for Ingska

Hi guys,

I am still kind of struggeling with my upgrade from php4 to php5, my site is almost there but I can't figure out how to set my http_referer.

This is how it looks like today:

<?php
 $_SESSION["referer"] .= $_SERVER['HTTP_REFERER'];
 $my_web = $_SESSION["referer"]
?>

The error message I get is "undefined variable: _SESSION and undefined index: referer".

I have tried to put isset() everywhere, but I can not figure out the soulution.

Any suggestions? :)

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Generated

Brief summary: the "undefined variable: _SESSION" and "undefined index: referer" errors came from using the session array before a session was started and from assuming HTTP_REFERER always exists. As experienced, missing initialization and a small syntax mistake in an if caused the parse error; 's later posts show the right direction (start the session, guard the variables).

Key fixes and modern best practices:

  • session_start() must run before any output (HTML/echo) so the $_SESSION superglobal is available.
  • Never concatenate onto a session key that might not exist. Either initialize it (or use the null-coalescing approach on modern PHP: $_SESSION['referer'] = ($_SESSION['referer'] ?? '') . ($_SERVER['HTTP_REFERER'] ?? '');) or keep referers as an array ($_SESSION['referers'][]) for easier handling.
  • Always check that $_SERVER['HTTP_REFERER'] is set before using it and validate it (for example with filter_var(..., FILTER_VALIDATE_URL)) — HTTP_REFERER is optional and user-controlled.

Reliability and security notes:

  • HTTP_REFERER is unreliable (may be stripped by browsers, proxies, or extensions) and can be spoofed; it should never be used for authentication or authorization decisions. For reliable navigation tracking, use explicit parameters, hidden form fields, or server-side routing logic instead of relying on the referer header.
  • Prefer storing structured data in session (arrays, timestamps) rather than building a single concatenated string.

Quick debugging tips:

  • For parse errors, look for mismatched parentheses/braces (that was the cause of the "unexpected '{'").
  • Enable development errors with error_reporting(E_ALL); ini_set('display_errors', 1); and inspect var_dump($_SESSION) or server logs to confirm values.
  • After the above fixes (as shown in the thread by ), the approach used by is robust for simple, non-security-critical referer tracking.

Recommended Answers

All 6 Replies

Member Avatar for cwarn23

Your script should be the following:

<?php session_start();
 $_SESSION["referer"] .= $_SERVER['HTTP_REFERER'];
 $my_web = $_SESSION["referer"]
?>

Also you will need to find another alternative to $_SERVER as it isn't always that accurate. The reason being is users can disable that information being sent.

Member Avatar for Ingska

Thanks for the suggestion, but it still doesn't work. I still get the same undefined variable/index errors :(

Member Avatar for cwarn23

Ow now I see. Another bug. Try this.

<?php session_start();
if (!isset($_SESSION['referer']) { 
    $_SESSION['referer']='';
    }
$_SESSION['referer'].=$_SERVER['HTTP_REFERER']."\n";
$my_web = $_SESSION['referer']
?>
Member Avatar for Ingska

I think it's something along these lines i need. But now i got an error saying "unexpexted '{' in line 2". When i take the '{' away, obviously I get an other error. I also tried to mix up the code a little, but i still can't get it right.

Member Avatar for cwarn23

OMG these bugs keep on coming. Did you hide any more for me to fix?

<?php session_start();
if (!isset($_SESSION['referer'])) { 
    $_SESSION['referer']='';
    }
$_SESSION['referer'].=(isset($_SERVER['HTTP_REFERER']))?($_SERVER['HTTP_REFERER']."\n"):'';
$my_web = $_SESSION['referer'];
?>

This time I tested it :)

Member Avatar for Ingska

Thank you so much! Now it work's perfectly :)

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