Parece um problema conhecido, consulte o link
Because of a PHP confirmed bug, filter_input( INPUT_SERVER, 'anything' ) would return null on some implementations of FCGI/PHP 5.4 ( and probably older versions as well ). https://bugs.php.net/bug.php?id=49184
Use workaround mentioned in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=730094:
if (filter_has_var(INPUT_SERVER, "SERVER_NAME")) {
$servername = filter_input(INPUT_SERVER, "SERVER_NAME",
FILTER_UNSAFE_RAW, FILTER_NULL_ON_FAILURE);
} else {
if (isset($_SERVER["SERVER_NAME"]))
$servername = filter_var($_SERVER["SERVER_NAME"],
FILTER_UNSAFE_RAW, FILTER_NULL_ON_FAILURE);
else
$servername = null;
}