All possible server variables:

ALL_HTTP
ALL_RAW
APPL_MD_PATH
APPL_PHYSICAL_PATH
AUTH_PASSWORD
AUTH_TYPE
AUTH_USER
CERT_COOKIE
CERT_FLAGS
CERT_ISSUER
CERT_KEYSIZE
CERT_SECRETKEYSIZE
CERT_SERIALNUMBER
CERT_SERVER_ISSUER
CERT_SERVER_SUBJECT
CERT_SUBJECT
CONTENT_LENGTH
CONTENT_TYPE
GATEWAY_INTERFACE
HTTPS
HTTPS_KEYSIZE
HTTPS_SECRETKEYSIZE
HTTPS_SERVER_ISSUER
HTTPS_SERVER_SUBJECT
INSTANCE_ID
INSTANCE_META_PATH
LOCAL_ADDR
LOGON_USER
PATH_INFO
PATH_TRANSLATED
QUERY_STRING
REMOTE_ADDR
REMOTE_HOST
REMOTE_USER
REQUEST_METHOD
SCRIPT_NAME
SERVER_NAME
SERVER_PORT
SERVER_PORT_SECURE
SERVER_PROTOCOL
SERVER_SOFTWARE
URL
HTTP_ACCEPT
HTTP_ACCEPT_ENCODING
HTTP_ACCEPT_LANGUAGE
HTTP_COOKIE
HTTP_HOST
HTTP_REFERER
HTTP_USER_AGENT
HTTP_X_FORWARDED_FOR
HTTP_X_FORWARDED_PROTO
HTTP_X_FORWARDED_PORT
HTTP_X_AMZN_TRACE_ID
HTTP_X_EC_HOT_HASH
HTTP_X_EC_UUID
HTTP_X_EC_SESSION_ID
HTTP_X_EC_POP
HTTP_X_HOST
HTTP_X_GATEWAY_LIST
HTTP_X_EC_INTERNAL_TRAFFIC_TYPE