.htaccess: how to restrict access to a single file by IP?

This will allow either someone from IP 127.0.0.1 or logged as a valid user. Stick it either in your config or .htaccess file.

    <Files learn.php>
        Satisfy any
        Order deny,allow
        Deny from all
        Allow from 127.0.0.1

        AuthType Basic
        AuthName "private"
        AuthUserFile /var/www/phpexperts.pro/.htpasswd
        AuthGroupFile /dev/null
        Require valid-user
    </Files>

IP Alone:

    <Files learn.php>
        Order deny,allow
        Deny from all
        Allow from 127.0.0.1
    </Files>

That definitely answers your question.

I think the directive needs to be:

Order deny,allow

for the answer above to work (at least for the IP Alone solution).