Simplify migration LegacyBridge example.

Rather than return null and then return the prior request, just send the request, and
fall back to the LegacyBridge if needed.

Author: jzohrab

migration.rst

@@ -275,16 +275,13 @@ could look something like this::
     $request = Request::createFromGlobals();
     $response = $kernel->handle($request);
 
-    /*
-     * LegacyBridge will take care of figuring out whether to boot up the
-     * existing application or to send the Symfony response back to the client.
-     */
-    $scriptFile = LegacyBridge::prepareLegacyScript($request, $response, __DIR__);
-    if ($scriptFile !== null) {
-        require $scriptFile;
-    } else {
+    if (false === $response->isNotFound()) {
+        // Symfony successfully handled the route.
         $response->send();
+    } else {
+        LegacyBridge::handleRequest($request, $response, __DIR__);
     }
+
     $kernel->terminate($request, $response);
 
 There are 2 major deviations from the original file:
@@ -297,10 +294,9 @@ Line 18
   it over. For instance, by replacing outdated or redundant libraries with
   Symfony components.
 
-Line 41 - 50
-  Instead of sending the Symfony response directly, a ``LegacyBridge`` is
-  called to decide whether the legacy application should be booted and used to
-  create the response instead.
+Line 41 - 46
+  If Symfony handled the response, it is sent; otherwise, the ``LegacyBridge``
+  handles the request.
 
 This legacy bridge is responsible for figuring out which file should be loaded
 in order to process the old application logic. This can either be a front
@@ -316,19 +312,22 @@ somewhat like this::
 
     class LegacyBridge
     {
-        public static function prepareLegacyScript(Request $request, Response $response, string $publicDirectory): ?string
+        public static function handleRequest(Request $request, Response $response, string $publicDirectory): ?string
         {
-            // If Symfony successfully handled the route, you do not have to do anything.
-            if (false === $response->isNotFound()) {
-                return null;
-            }
-
             // Figure out how to map to the needed script file
-            // from the existing application and possibly (re-)set
-            // some env vars.
-            $legacyScriptFilename = ...;
+            // from the existing application.
+            function getScript($requestPathInfo) { ... }
+
+            $p = $request->getPathInfo();
+            $legacyScriptFilename = getScript($p);
+
+            // Possibly (re-)set some env vars (e.g. to handle forms
+            // posting to PHP_SELF):
+            $_SERVER['PHP_SELF'] = $p;
+            $_SERVER['SCRIPT_NAME'] = $p;
+            $_SERVER['SCRIPT_FILENAME'] = $legacyScriptFilename;
 
-            return $legacyScriptFilename;
+            require $legacyScriptFilename;
         }
     }