Version Mayhem

Start by deleting every php version

sudo apt-get purge php7.*
sudo apt-get autoclean
sudo apt-get autoremove

Then follow a suitable guide and install the correct version.

Switching Active Version

Enabling and disabling a specific version.

For plain php;

# disable
sudo a2dismod php7.0
 
# enable
sudo a2enmod php7.2

For php-fpm;

# disable
sudo service phpx.x-fpm stop
 
# enable
sudo service phpx.x-fpm start

Then rerun web server

sudo service apache2 restart
sudo service nginx restart

Plugins

When installing plugins don't just install it by it's name. You must specify the php version as well. For example when trying to install the php-xml plugin. You must run;

sudo apt install php-xml

This would install the plugin for the most recent version of php. Which may or may not be the version you have running. So, first check the php version you have running using php --version. Then install the package that's relevant to it.

# for php7.4
sudo apt install php7.4-xml

Testing

Create the directory /var/www/php-test and place an index.php file with;

<?php
  phpinfo();
?>

Config Location

Usually there's multiple php config files that exist. So you must figure out which file you're using. Then again, you could even be using multiple at the same time.

However, if you're running both apache and nginx at the same time. There's a possibility that the two are using different php.ini's.

First create a php-test file. Then serve it with the preferred web server.

Apache

Create a new conf file at /etc/apache2/sites-available and point it towards the test file.

Remember to route the traffic if you're using an nginx reverse proxy to serve apache requests with nginx requests. If so, you'll need to customize the apache config at /etc/nginx as well.

Nginx

Create a config file at /etc/nginx with the usual php stuff. Then point it towards the test file.

Troubleshooting

ERROR - 413

The error 413 files too large is usually caused by the web server or php.

Apache

Increase the LimitRequestBody. It can be used in any virtualhost or location directive. Check here (opens in a new tab) for more info. You need to convert whatever request size you need into bytes. Use this (opens in a new tab) to convert. Remeber to restart apache afterwords.

<Directory "/var/www/site">
	LimitRequestBody  5242880
</Directory>

Nginx

Increase the client_max_body_size at either /etc/nginx/nginx.conf or the site config file. Afterwords remember to restart nginx.

client_max_body_size 2M;

PHP

First figure out which php .ini file is used here. Change these parameters. Afterwords remember to restart the web server and php-fpm.

upload_max_filesize = 100M
post_max_size = 100M