I faced a problem last day while updating the header_php.php file for one of the pages I created in Zen Cart. Even after making changes in the header_php file, the changes were not reflected when I viewed the page in browser.
The header_php.php file is present in
\includes\modules\pages\{page_name}\
where {page_name} should be replaced by the name of the page.
The problem was I kept backups of older header_php files on server in the same directory. And Zen Cart automatically includes all the files present in this folder. Hence my updates were overwritten by the old file.
Solution :: Never keep backup files in \includes\modules\pages\{page_name}\