I've got a small question : for instance, i've got a domain: www.mydomain.com, on host in httpdocs this site is within folder test. so how do i redirect to gain access to this site by domain: www.mydomain.com rather than www.mydomain.com/test

Do you can get the Apache configuration? You can easily customize the DocumentRoot property from the vhost for www.mydomain.com. You pointed out a number though, so I am speculating not. Here's the following best factor: you can include a mod_rewrite rule to .htaccess within the cause of www.mydomain.com, presuming your host enables this:

RewriteEngine on
RewriteRule ^(.*)$ /test/$1

Inside your .htaccess file within the DocumentRoot put:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /test/$1 [QSA]
</IfModule>

This makes all of the Web addresses for example www.foo.com/hello.html be internally rewritten to www.foo.com/test/hello.html and voila you'll achieve preferred effect.