I was running Piwigo v13.8 under php8.1 with only Cookies Consent and LocalFiles Editor plugins enabled. Tried using update function from Piwigo but just received a blank screen when upgrade.php tried to run.
So I went to the root of my website was getting the mysql_connect issue. I remembered this happening to me before but completely forgot how to fix it.

Long story short, using these forums, I did the following:
1. Copied all Piwigo 14 file over mine.
2. Updated the dblayer to 'mysqli' in local/config/database.inc.php.
3. Ran website from root and the upgrade page came up. I knew I was on the path to success.
4. Ran the upgrade without any problems. Update was extremely fast. I did notice the upgrade disabled 'Cookies Consent' plugin.
5. Enabled 'Cookies Consent' plugin without issue.

Thank you to everyone on Piwigo's team for all their hard work during this update!

I appreciate it very much!