First change the URL for access.
Then when the URL is reached, one should be asked for a valid username or password before accessing the actual phpmyadmin interface.
Also, To increase the odds of a phpmyadmin URL not being indexed by any search engine make sure it is told not to index. Modify the Phpmyadmin code and make sure that:
header("X-Robots-Tag: noindex, nofollow", true);
is added before html output (before any print or echo statements). Adding it near other lines containing "header" is ok.
Alternatively, you can edit the html output and include the robots meta tag in it.
See: http://www.robotstxt.org/meta.html for info.