I built a website for a former employer in 2012 using zenphoto, tons of customization on the theme, and working great for years, but a few weeks ago something changed on the server and totally broke the installation. I figured it was time to finally upgrade from the old 1.4.6 install to the newest one 1.5.6, as PHP is forced to be 7.3 now the host, so needed a new version for the best compatibility.

I managed to get the installation upgraded, and was able to get into the back-end just fine, everything is there and looks functional from that end, but whenever I try to actually view the website, zenphoto keeps booting back into "reinstall" mode. It runs and gives me green check marks on everything post reinstall, but then just only gives me the option to manage the website afterwards, but I cannot get the actual site to save my life. Every time I view it goes back to reinstall, I've deleted setup files to no avail.

Oddly enough when I google this I literally get dozens of other websites stuck in the same exact state. Like this

Any suggestions? Thanks in advance!

Those sites you see are almost all older ones that are not properly maintained (e.g not updated or otherwise cared for, often they are also rather empty). On those older versions setup would always started to run automatically even not an admin with full rights if it detects changes on the server like a new php version (it will still do that if loggedin). Now it should only should block the site if logged in as a full admin, unless you install a new version.

It is hard to tell what happens on your install from here. In any case you should not delete the setup files as they might be required. Zenphoto protects them after running automatically.
You really need to review your error logs. Both the server php error log and Zenphoto's own debug log. You might have some file/folder permission issues that can cause all sorts of issues.
It might also be that you didn't strictly follow the upgrade process. You jumped a few versions which we neither test nor guarantee to work properly.

Also try force clearing your browser cache. That is quite persistent on some nowadays and leads to a lot issues sometimes. (that's probably not it but doesn't hurt…)

Since you say "tons of customization on the theme" I really hope you tested on a staging site before updating the live site of your client as there might things require maintenance as there have been several rather major updates. You should review you logs regarding that as well.

PS: Yes, the upgrade was overdue since 1.4.6 is from 2014! ;-)

Thanks for the reply, I just double checked the logs, PHP log is just a huge thread of failures to load dynamic libraries from php5.4, the server is running 7.0.33 (I tried pushing it to 7.3 multiple times, but it won't stick, trying to get the admin to force it and turn off whatever php 5.4 libraries are trying to load). Stupid bluehost upgraded to a new cpanel, which is missing a good 80% of the controls in the old version.

The zenphoto error log only shows one thing:

Methods with the same name as their class will not be constructors in a future version of PHP; ThemeOptions has a deprecated constructor in /home1/../public_html/themes/dl_zenpage/themeoptions.php on line 12

The debug log shows this one error:

EXCEPTION (8192): Methods with the same name as their class will not be constructors in a future version of PHP; google_analyticsOptions has a deprecated constructor in /home1/.../public_html/plugins/google_analytics.php on line 66

require_once called from setup_pluginOptions.php [22]

google analytics is disabled BTW.. mod_rewrite is off for now as it was causing problems with the setup before and Setup did not detect a working mod_rewrite facility in options.

I did the upgrade following the instructions several times, deleting and replacing index.php, zp-core folder, and the default themes (running a custom copy which I kept around)

I remember setting it up locally on MAMP back in 2012, developing it there, the uploading to the web, but I'm pretty sure that computer is long gone by now! So just left with the online version.

My next option is making a copy of public_html as a backup, then totally cleaning it out and doing a fresh as fresh can get install of the new version, including a new MySQLi database, then a clean zenphoto install. My question is, what do I definitely need to keep to get the site running again?

Was going to save and transfer over:

1- My custom theme from themes folder (I spent several months building this)
2- A recent zenphoto database backup file (will this be enough to bring everything back with a blank new database?)
3- Uploaded folder
4- Albums Folder
5- Cache folder? (would save from having to re-generate smaller images again)

Does that sound like it would be enough to get everything transferred over to a clean install?

I think the answer is in the error message. In your custom theme folder (dl_zenpage) find the themeoptions.php file and on line 12 change function ThemeOptions() to function __construct().
A similar change is needed in google_analitics.php line 66.

PHP 7+ should be fine actually.

The constructor issue in your log is about deprecation and will still work but will break sometime in the future when PHP gets an update. So it should be fixed. But as fretzl say its an easy fix. But regarding your theme I strongely recommed testing this with PHP 7 and reviewing the PHP 5 to 7 changes on php.net. You might have other things that already break in your theme.

MAMP or similar is fine for devloping, we use MAMP as well.

To your questions:

1- My custom theme from themes folder (I spent several months building this)

Yes, you of course need that.

2- A recent zenphoto database backup file (will this be enough to bring everything back with a blank new database?)

The backup must be of the same version you try to re-setup. There are database changes between versions that otherwise may cause trouble

3- Uploaded folder

If you have content in there, yes. Otherwise an empty folder will do.

4- Albums Folder

Yes, you need these. Please carefull review the user guide about moving sites (which is technically what you do) as the order of doing things regarding the albums is very important to not loose content:
https://www.zenphoto.org/news/installation-and-upgrading/#--moving-existing-installations

5- Cache folder? (would save from having to re-generate smaller images again)

You can if you don't want to regenerate. technically not required as Zenphoto will regenerate when needed.

Since its not your own site, I hope you can also educate thte owner that CMS powered sites are not an "Install and forget" type and require constant maintenance. Not only system updates but of course dependencies in custom code.