I am running PHP 5.3.2 and Apache 2.2.14. Using header(), Basically send "ETagx" I recieve the header within the response, but when I send "ETag", the right header title, I recieve nothing.

header('ETagx: "33653a-4831d8249af80"')

works, while

header('ETag: "33653a-4831d8249af80"')

doesn't. It is possible to configuration option in php.ini, or most of the Apache configuration files that may affect this?