SOLVED On the off chance that anybody else runs into this, I realized that I was using suphp as my PHP handler on my previous server (and on the PHP 5.6 on the new one), and the new server had cgi set as the default PHP handler for PHP 7.3. As soon as I set to the suphp, everything worked perfectly!
I recently migrated an EE 5.3.2 site from a CentOS 6 server to a CentOS 7 server, and everything works fine except for a page that I have password protected using EE’s built-in HTTP Authentication option in the template settings. It was working fine on the previous server, but it just is stuck in an endless loop of not accepting login credentials on the new server.
Both the old server and the new one were running PHP 7.3 and have the exact same php.ini file. Neither were/are using PHP-FPM. The weird thing is that if I roll back to PHP 5.6 on the new server HTTP Authentication works fine on the page. Unfortunately, PHP 5.6 is past EOL, so that’s obviously not a solution to the problem. I compared the php.ini file for PHP 5.6 to the one for PHP 7.3, and there were no obvious glaring differences.
Anyone else experience HTTP Authentication not working in PHP 7.3? If so, have you found a solution? Thanks!