It returns a list of response headers sent

Example

<?php
 setcookie("time",time());
 header('Content-type: text/plain');
 echo "<pre>";
 print_r(headers_list());
 ?>

Output

<pre>Array
 (
 [0] => X-Powered-By: PHP/5.3.3-1ubuntu9.10
 [1] => Set-Cookie: time=1337938449
 [2] => Content-type: text/plain
 )