After upgrading PHP version get Cannot Access Filesystem error on WordPress with nginx

Supporting an old WordPress on Ubuntu 16.04 it was running PHP 7.3 and we really needed to upgrade it to 7.4. But when we did and switched the fastcgi_pass to point to the new version, the site choked.

All webpages suddenly asked for username and password/FTP details to complete the operation and then said that it couldn’t access the filesystem.

I checked the php7.4-fpm.log and there were no errors, likewise in the nginx error.log. It seems like permissions, but they all seemed to check out in /run/php/php7.4-fpm.sock

Curiously, the previous developer was running PHP 7.3 from /run/php-wp-fpm.sock, and not in the /run/php/ where it is created when you install 7.3 or 7.4. Why would they do that?

After some hours of reading every article in the world on this (there’s a lot!) I thought I’d try and run it in /run/ rather than in /run/php/

So altered the /etc/php/7.4/fpm/pool.d/www.conf like this – altering the config and restarting the service creates the new sock file in that folder.

listen = /var/run/php7.4-fpm.sock
listen.owner = www-data
listen.group = www-data
user = web
group = www-data

And then edit the /etc/nginx/sites-available/my-website.com.conf file to match:

fastcgi_pass unix:/run/php7.4-fpm.sock;

Then we restart both fpm and nginx:

systemctl restart php7.4-fpm
service nginx restart

And voila! It worked. No idea why but I guess this is why the previous developer did this!

For reference this is what I did to install PHP on the creaky old Ubuntu version before this:

sudo apt-get update
sudo apt-add-repository ppa:ondrej/php
(this repo may not have what you need, you could try ppa:jczaplicki/xenial-php74-temp but this it at your own risk)
sudo apt install -y php7.4 php7.4-cli php7.4-common php7.4-fpm
sudo apt install -y php7.4-mysql php7.4-dom php7.4-simplexml php7.4-ssh2 php7.4-xml php7.4-xmlreader php7.4-curl  php7.4-exif  php7.4-ftp php7.4-gd  php7.4-iconv php7.4-imag
sudo apt install -y php7.4-mysqli php7.4-pdo  php7.4-sqlite3 php7.4-ctype php7.4-fileinfo php7.4-zip php7.4-exif
sudo update-alternatives --config php (choose 7.4)
(edit your config files as above)
systemctl restart php7.4-fpm
service nginx restart

I’m a complete Linux/nginx novice, so this has truly been a nightmare for me, I hope this helps at least one person!