Well.
Finally, I’ve got to setup correctly my apache configuration to work fine with user_dir module. In my **/etc/apache2/mods-enabled, I’ve commented these lines:
#<IfModule mod_userdir.c>
# <Directory /home/*/public_html>
# php_admin_flag engine Off
# </Directory>
#</IfModule>
The route for my web sites in localhost is now: http://localhost/~user/site, with ~ before user name.
Thanks for your help.