Drupal configuration on non root url (how to change $base_url)? sorry for my english, I'm french

ok, that's I want to do is to install my drupal site on a url like : 
www.mymachine.fr/mydrupal/

and the problem is that when I go this url, 
he dispatch me to www.mymachine.fr/core/install.php
instead of www.mymachine.fr/mydrupal/core/install.php

so the install doesn't work 

My conf is like this : debian server

in drupal file "sites.php", I have this :

$sites['mymachine.fr.mydrupal'] = 'mydrupal';

on /etc/hosts, I add this line  : 
    127.0.0.1    mymachine.fr.mydrupal

on /etc/apache2/sites-available/000-default.conf : 

&lt;VirtualHost *:80&gt;
    ServerName mymachine.fr.mydrupal
    DocumentRoot /var/www/mydrupal/CORE-DRUPAL8-WEB/web

    ServerAdmin webmaster@localhost

    # Log configuration
    ErrorLog ${APACHE_LOG_DIR}/drupal-error.log
    CustomLog ${APACHE_LOG_DIR}/drupal-access.log combined

    &lt;Directory "/var/www/mydrupal/CORE-DRUPAL8-WEB/web"&gt;
            Options FollowSymLinks
            AllowOverride All
            Require all granted
    &lt;/Directory&gt;

&lt;/VirtualHost&gt;


SO I don't know if it's drupal configuration or apache configuration to change $base_url from drupal site ...
thanks a lot
If your drupal installation resides on the webserver in a subfolder of the webroot for www.mymachine.fr, you can set the base path in the drupal site's .htaccess file.  


Find the line that reads # RewriteBase /drupal 
Uncomment it by deleting the "#" RewriteBase /drupal
Edit it to reflect your subpath/subfolder # RewriteBase /mydrupal


That block should look like this after the edits.

# Modify the RewriteBase if you are using Drupal in a subdirectory or in a
# VirtualDocumentRoot and the rewrite rules are not working properly.
# For example if your site is at http://example.com/drupal uncomment and
# modify the following line:
RewriteBase /mydrupal


Then, reset your .conf (vhost entry) to reflect just the top level website (code sample below resets to default .conf setup):

&lt;VirtualHost *:80&gt;
  ServerName mymachine.fr.mydrupal
  DocumentRoot /var/www

  ServerAdmin webmaster@localhost

  # Log configuration
  ErrorLog ${APACHE_LOG_DIR}/error.log
  CustomLog ${APACHE_LOG_DIR}/access.log combined

  &lt;Directory "/var/www"&gt;
      Options FollowSymLinks
      AllowOverride All
      Require all granted
  &lt;/Directory&gt;
&lt;/VirtualHost&gt;


This setup presumes that your top level domain does not have any redirection rules that forward/process all requests (such as the rules in an out-of-the-box Drupal or Wordpress .htaccess for processing requests with index.php)  If your top level domain has such a redirect rule in place, you may need to add another rule to exclude your mydrupal subdirectory.