Update $MYSITE to use HTTP_HOST

Author: markrandall

include/site.inc

@@ -244,7 +244,14 @@ if (!isset($_SERVER["HTTPS"]) || $_SERVER["HTTPS"] != "on") {
     $proto = "https";
 }
 
-if ($_SERVER["SERVER_PORT"] != '80' && $_SERVER["SERVER_PORT"] != 443) {
+$hostHeader = $hostHeader = $_SERVER["HTTP_HOST"] ?? null;
+if ($hostHeader
+    /* incoming hostname validation logic */
+    && preg_match('/^([a-z0-9]([a-z0-9-]*[a-z0-9])?\.)*[a-z0-9]([a-z0-9-]*[a-z0-9])?(?::\d{1,5})?$/i', $hostHeader)
+) {
+    $MYSITE = $proto . '://' . $hostHeader . '/';
+    $msite = 'https://' . $hostHeader . '/';
+} elseif ($_SERVER["SERVER_PORT"] != '80' && $_SERVER["SERVER_PORT"] != 443) {
     $MYSITE = $proto . '://' . $_SERVER["SERVER_NAME"] . ':' . (int)$_SERVER["SERVER_PORT"] . '/';
     $msite = 'http://' . $_SERVER["SERVER_NAME"] . ':' . (int)$_SERVER["SERVER_PORT"] . '/';
 } else {