For some reason a call to header() causes an internal server error for me. I'm using PHP5 and use mod_rewrite extensively in this script (if that helps). Here is the code (sort of):
<?php 
include 'core/initialize.php'; // Loads the core class (and session manager class)
    if($_GET['reset'] == 'true')
    {
         $core->Session->Visits = 0;
         header('Location', 'index.html');
         # header('X-Test', 'wtf'); // causes the error too :(
    }
    if(isset($core->Session->Visits)) $core->Session->Vists += 1;
    else $core->Session->Visits = 0;
    echo "Previous Visits: {$core->Session->Visits} (<a href='index.html?reset=true'>Reset</a>)";
?>
My .htaccess file looks like this:
# Start up the rewrite engine
Options +FollowSymLinks
RewriteEngine on
RewriteRule  ^(.*)$ navigator.php?nav=$1&%{QUERY_STRING} [NC]  
 
     
    