Sending machine hostname as a header in Apache
You need to pass an Environment Variable using mod_env:
PassEnv HOSTNAME
Header set X-Hostname "%{HOSTNAME}e"
You can set the value of HOSTNAME through the envvars file (mine is /etc/apache2/envvars)
Also, if you're using PHP you can use environment variables
Untested, but how about passing the line in via the init (or other startup) script? Something like:
/path/to/httpd -c "Header set X-Hostname $HOSTNAME"
If you're using a RH-flavour distro you might be able to squeeze this into /etc/sysconfig/httpd
or similar to avoid editing the init script.