This is the error message: Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /var/autofs/home/s_home/jp245/public_html/php/index.html:1) in /var/autofs/home/s_home/jp245/public_html/php/index.html on line 3

**just to let you know, I've altered some files that allow me to run php code in html files**

What does this mean? Can I fix this problem?

<?
session_start();
if ($_SESSION['myusername'] != NULL ) {
echo "<META HTTP-EQUIV=\"refresh\" content=\"0;
URL=nav.php \">";
}
?>