Yii: .htaccess and urlManager for separate backend

Question

I'm having a hard time to configure my .htaccess and the urlManager in a Yii project to have the frontend in http://www.example.com and the backend in http://www.example.com/backend with the following folder structure. Any help is welcome. Thanks.

/assets
/backend
   /controllers
   /config
      main.php
   /models
   /views
/common
   /models
/protected
   /controllers
   /config
      main.php
   /models
   /views 
.htaccess
backend.php
index.php

Solution: after the great help of @bool.dev everything it's working, so I'm adding here every needed final file. In the frontend I'm using path format for the url and hiding the index.php

/backend/config/main.php

$backend=dirname(dirname(__FILE__));
Yii::setPathOfAlias('backend', $backend);
return array(
'basePath' => $backend,

'controllerPath' => $backend.'/controllers',
'viewPath' => $backend.'/views',
'runtimePath' => $backend.'/runtime',

...);

/protected/config/main.php

'urlManager'=>array(
    'urlFormat'=>'path',
    'showScriptName'=>false,
    'rules'=>array(
            '<controller:\w+>/<id:\d+>'=>'<controller>/view',
            '<controller:\w+>/<action:\w+>/<id:\d+>'=>'<controller>/<action>',
            '<controller:\w+>/<action:\w+>'=>'<controller>/<action>',
    ),
),

.htaccess

Options +FollowSymLinks
IndexIgnore */*
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteBase /yii/example/
RewriteRule backend backend\.php [T=application/x-httpd-php]

# if a directory or a file exists, use it directly
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

# otherwise forward it to index.php
RewriteRule . index.php
</IfModule>

backend.php

$yii=dirname(__FILE__).'/../../yii/framework/yii.php';
$config=dirname(__FILE__).'/backend/config/main.php';
require_once($yii);
Yii::setPathOfAlias('common', dirname(__FILE__).DIRECTORY_SEPARATOR.'common');
Yii::createWebApplication($config)->run();

index.php

$yii=dirname(__FILE__).'/../../yii/framework/yii.php';
$config=dirname(__FILE__).'/protected/config/main.php';
require_once($yii);
Yii::setPathOfAlias('common', dirname(__FILE__).DIRECTORY_SEPARATOR.'common');
Yii::createWebApplication($config)->run();

Answer 1

According to this wiki article by Qiang, you could make the following changes and it should work:

// backend.php:
require('path/to/yii.php');
Yii::createWebApplication('backend/config/main.php')->run();

Then in your backend's config (i.e backend/config/main.php):

$backend=dirname(dirname(__FILE__));
$frontend=dirname($backend);
Yii::setPathOfAlias('backend', $backend);

return array(
    'basePath' => $backend,

    'controllerPath' => $backend.'/controllers',
    'viewPath' => $backend.'/views',
    'runtimePath' => $backend.'/runtime',

    'import' => array(
        'backend.models.*',
    ),
    // ... other configurations ...
);

But for this to work we need the main .htaccess to route example.com/backend to backend.php, which i haven't figured out yet.

Edit:
Just figured out:

RewriteEngine On
RewriteBase /projectroot/
RewriteRule backend backend\.php [T=application/x-httpd-php]

The RewriteBase was important for me, as the backend.php was not found when i hadn't given the correct projectroot, basically it should be the directory where you have the entry script backend.php.