use application.pantheon.php to define WP_HOME

and make it available for future application.php modifications

config/application.pantheon.php

@@ -0,0 +1,40 @@
+<?php
+/**
+ * Pantheon platform application settings.
+ *
+ * IMPORTANT NOTE:
+ * Do not modify this file. This file is maintained by Pantheon.
+ *
+ * Site-specific modifications belong in config/application.php, not this file.
+ * This file may change in future releases and modifications would cause
+ * conflicts when attempting to apply upstream updates.
+ */
+
+use Roots\WPConfig\Config;
+use function Env\env;
+
+if ( isset( $_ENV['PANTHEON_ENVIRONMENT'] ) && 'lando' !== $_ENV['PANTHEON_ENVIRONMENT'] ) {
+    // We can use PANTHEON_SITE_NAME here because it's safe to assume we're on a Pantheon environment if PANTHEON_ENVIRONMENT is set.
+    $sitename = $_ENV['PANTHEON_SITE_NAME'];
+    $baseurl = $_ENV['PANTHEON_ENVIRONMENT'] . '-' . $sitename . '.pantheonsite.io';
+
+    $scheme = 'http';
+    if ( isset( $_SERVER['HTTPS'] ) && 'on' === $_SERVER['HTTPS'] ) {
+        $scheme = 'https';
+    }
+
+    // Define the WP_HOME and WP_SITEURL constants if they aren't already defined.
+    if ( ! env( 'WP_HOME' ) ) {
+        // If HTTP_HOST is set, use that as the base URL. It's probably more accurate.
+        if ( isset( $_SERVER['HTTP_HOST'] ) )     {
+            $baseurl = $_SERVER['HTTP_HOST'];
+        }
+
+        $homeurl = $scheme . '://' . $baseurl;
+        Config::define( 'WP_HOME', $homeurl );
+
+        if ( ! env( 'WP_SITEURL' ) ) {
+            Config::define( 'WP_SITEURL', $homeurl . '/wp' );
+        }
+    }
+}