To make a file writeable by the web server user on CentOS you need to set both the traditional permissions and the SELinux security context for that file.
The web server user in CentOS is called
chgrp apache /var/www/my-file chmod g+rw /var/www/my-file semanage fcontext -a -t httpd_sys_rw_content_t /var/www/my-file restorecon -R -F /var/www/my-file
Relative paths from your current directory are acceptable for
chmod, but you have to use the full absolute path for the SELinux commands.