I'm afraid there is no MOST SECURE one for download, especially for a non-common purpose such as a custom site, in comparison to Invision Board, for example. ... meaning you won't find a good, secure one for download.
The basics to PHP security are to find good, expensive hosts that have the ultamite firewalls, etc.
The second step is YOUR job with the PHP.
I just recently wrote the most secure admin protected pages and login that I've ever done. It doesn't use MySQL to store the pwds since only I will be accessing it, but some good advice...
use SSL. This costs more $$$.
encrypt passwords two or three times. I encrypt my already-"encrypted" md5 with crypt(). This takes longer but is more secure.
use plenty of protection amongst admin pages, and ALWAYS be sure to secure against sql injection. I recommend using Axilant's script (in CODE forum) and use plenty of cookies, session variables, and even write your own logs.
One of the neat security features of one of my sites is that upon a detected or sensed security breach, it shuts down the entire site from user access, notifies me by e-mail, blocks ALL ip addresses but mine, ONLY allows Firefox browser, changes my admin password and sends an encrypted email to me of the new password, only lets ME login to the admin panel from MY computer with MY browser.
Plus some.
Hope I've helped,
cheers,
-Matt