Steps to reproduce:
Run PHP as FCGI
Try to POST a grade from an external tool to Moodle
Nothing is reported

This is due to the fact that getheaders() used in service.php only works when mod_php is used via Apache.

My solution was to use a .htaccess with this in it:
RewriteEngine on
RewriteRule .* - [E=HTTP_AUTHORIZATION:%

,L]

And insert the following code at the very top of the page

if (!function_exists('getallheaders'))
{
function getallheaders()
{
$headers = '';
foreach ($_SERVER as $name => $value)
{
if (substr($name, 0, 5) == 'HTTP_')

}

$headers['Authorization'] = $_SERVER['HTTP_AUTHORIZATION'];

return $headers;
}
}