the headers already sent error occurs when there is some sort of output on the page before the header() or session_start() functions are used.
if you look closely at the error, this section:
output started at /home/customel/public_html/header.php:6
is reporting that the output is at line 6 of the header.php. Ussusally you will find the problem on the previous line though.
In most cases, the problem is caused by a blank space between a closing php tag and the next opening tag:
if you are not able to spot the problem, send a copy of your page in a zip archive.