correctly determine protocol from port and configuration in PHP

Fixes: #1283

airtime_mvc/build/airtime.example.conf

@@ -33,6 +33,17 @@
 #                    on your webserver, relative to the base_url.
 #                    The default is /.
 #
+# force_ssl:         Use HTTPS for all API calls and internal links,
+#                    even if the web server is not operating on port
+#                    443. This is useful for working behind a reverse
+#                    proxy.
+#                    The default is False.
+#
+# protocol:          Set the specific protocol if required. This is
+#                    useful when using http on port 443. Mutually
+#                    exclusive with force_ssl.
+#                    Default is empty.
+#
 # cache_ahead_hours: How many hours ahead of time the Airtime playout
 #                    engine (pypo) should cache scheduled media files.
 #                    The default is 1.
@@ -54,6 +65,7 @@ base_url = localhost
 base_port = 80
 base_dir = /
 force_ssl =
+protocol = 
 cache_ahead_hours = 1
 airtime_dir =
 station_id =