when I access http://mydomain.com it’s ok.
but when I access http://www.mydomain.com It’s show text like “Apache is functioning normally” .

how to fix it?

Edit the vhost file for mydomain.com and add the following inside <VirtualHost>:

ServerAlias www.mydomain.com

Save the file, do a configtest:

sudo apachectl -t

Reload Apache:

sudo service apache2 reload

when I access mydomain.com and hover mouse point to link in my wordpress website

Login to WP-Admin > Settings > General and set the WordPress Address and Site Address fields.

hello my dear friends
i saw this question in google search, you can check this link for resolve this problem:
https://wiki.serversetup.co/2014/12/apache-functioning-normally-directadmin/