I have defined such virtual host on a local Apache server.
<VirtualHost *:80>
ServerAdmin webmaster@dummy-host2.example.com
DocumentRoot "c:/wamp/www/yii/frontend/web"
<Directory "c:/wamp/www/yii/frontend/web">
AllowOverride all
Require all granted
</Directory>
Alias /admin "c:/wamp/www/yii/backend/web"
<Directory "c:/wamp/www/yii/backend/web">
AllowOverride all
Require all granted
</Directory>
ServerName florist
ErrorLog "logs/yii-error.log"
CustomLog "logs/yii-access.log" common
</VirtualHost>
URL http://florist/admin works correctly and processed by /backend/web/index.php.
But URL http://florist/admin/login have processed by /frontend/web/index.php
I found this out when I placed exit() function with appropriate message in appropriate index.php
What's wrong may be in the alias?