.htaccess

<IfModule mod_rewrite.c>
    Options +FollowSymlinks
    RewriteEngine On
</IfModule>
<IfModule mod_rewrite.c>
    # если /admin, то в backend
    RewriteCond %{REQUEST_URI} ^/(admin)
    RewriteRule ^admin/assets/(.*)$ backend/web/assets/$1 [L]
    RewriteRule ^admin/css/(.*)$ backend/web/css/$1 [L]
    RewriteRule ^admin/js/(.*)$ backend/web/js/$1 [L]
    RewriteRule ^admin/libs/(.*)$ backend/web/libs/$1 [L]
    RewriteRule ^admin/img/(.*)$ backend/web/img/$1 [L]
    RewriteRule ^admin/files/(.*)$ backend/web/files/$1 [L]
    RewriteRule ^admin/plugins/(.*)$ backend/web/plugins/$1 [L]
    RewriteCond %{REQUEST_URI} !^/backend/web/(assets|js|css|js)/
    RewriteCond %{REQUEST_URI} ^/(admin)
    RewriteRule ^.*$ backend/web/index.php [L]
    RewriteCond %{REQUEST_URI} ^/(assets|css|js|images)
    RewriteRule ^assets/(.*)$ frontend/web/assets/$1 [L]
    RewriteRule ^css/(.*)$ frontend/web/css/$1 [L]
    RewriteRule ^js/(.*)$ frontend/web/js/$1 [L]
    RewriteRule ^images/(.*)$ frontend/web/images/$1 [L]
    RewriteRule ^(.*)$ frontend/web/$1 [L]
    RewriteCond %{REQUEST_URI} !^/(frontend|backend)/web/(assets|css|js)/
    RewriteCond %{REQUEST_URI} !index.php
    RewriteCond %{REQUEST_FILENAME} !-f [OR]
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule ^.*$ frontend/web/index.php
</IfModule>