Authorization header and apache_request_headers function

Authorization header and apache_request_headers function

I've been on a journey to getting apache_request_headers() working on my
server. I have upgraded to the latest stable of PHP 5.4 and changed my PHP
handler to FastCGI as this allows you to run the apache_request_headers()
function. I'd rather not run PHP as an apache module due to permission
issues.
Everything works fine with my new set-up but the only issue is that
apache_request_headers() does not seem to pick up the "Authorization"
header which I require for my OAuth 2 server.
The header I am sending is:
Authorization: Bearer xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
However, if I send the following header (or anything other than
'Authorization'), it works:
X-Authorization: Bearer xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Frustrating... Any ideas on how I can get this working?