Please use to below command

sudo update-alternatives --config php

After run above command select the PHP version that you need to use.

Press to keep the current choice[*], or type selection number: For example 2

After switching below command used to restart the PHP and Nginx server.

sudo service nginx restart
sudo service php7.1-fpm or php7.2-fpm  restart

Please try above code. If you have any help let me know.

I have the same problem. You forgot to change the default sockets from fastcgi_pass unix:/var/run/php-fpm/php-fpm.sock; to fastcgi_pass unix:/var/run/php/php5.6-fpm.sock; - you can find the configuration file at /etc/nginx/sites-available/default.

Before:

fastcgi_pass unix:/var/run/php-fpm/php-fpm.sock;

After:

fastcgi_pass unix:/run/php/php5.6-fpm.sock;

Interactive switching mode

sudo update-alternatives --config php
sudo update-alternatives --config phar
sudo update-alternatives --config phar.phar

Manual Switching

From PHP 5.6 => PHP 7.1

Default PHP 5.6 is set on your system and you need to switch to PHP 7.1.

Apache:

$ sudo a2dismod php5.6
$ sudo a2enmod php7.1
$ sudo service apache2 restart

Command Line:

$ sudo update-alternatives --set php /usr/bin/php7.1
$ sudo update-alternatives --set phar /usr/bin/phar7.1
$ sudo update-alternatives --set phar.phar /usr/bin/phar.phar7.1

From PHP 7.1 => PHP 5.6

Default PHP 7.1 is set on your system and you need to switch to PHP 5.6.

Apache:

$ sudo a2dismod php7.1
$ sudo a2enmod php5.6
$ sudo service apache2 restart

Command Line:

$ sudo update-alternatives --set php /usr/bin/php5.6

Source

Okay, you want to run multiple PHP version through nginx, the configure file should includes the specific path where you put your PHP scripts in different version or extension name.

However, I would like to explain the SF question link given in your post.

That answer is giving you a way to modify the conf of your nginx, which assumes that questioner has background in nginx.

By giving a specific port in conf, nginx will make the script executed through that FastCGI channel.

Let's pretend you have installed different PHP version in your server, and you have modified the port in php-fpm.conf.

You wish all php file executed in version 5.5.0 and php2 file executed in 5.6.0.

Example config of nginx as follows,

    listen       8080;
    server_name  localhost;

    root   html;

    location / {
        index  index.php index.php2;
    }

    location ~ \.php$ {
        fastcgi_pass   127.0.0.1:9000;
        fastcgi_index  index.php;
        fastcgi_param  SCRIPT_FILENAME  "${document_root}${fastcgi_script_name}";
        include        fastcgi_params;
    }

    location ~ \.php2$ {
        fastcgi_pass   127.0.0.1:9001;
        fastcgi_index  index.php;
        fastcgi_param  SCRIPT_FILENAME  "${document_root}${fastcgi_script_name}";
        include        fastcgi_params;
    }

In this case, php is processed through port 9000 and php2 goes to 9001

This is jsut a simple example, for advanced you can make two different folders to store php files, for example, /home/php55 and /home/php56, then edit your nginx.conf.

FOR YOUR EDITED QUESTION

If you want to try adding different servers to process the scripts in different version, sure you can do that, because nginx can be a load balancer, it can deal with this kind of problem as well.

First Application

server{
listen 80;
server_name php1.localhost; 

root   /home/www/php5.5;

location / {
  index  index.php;
}

location ~ \.php$ {
  fastcgi_pass   127.0.0.1:9002; (You can change it to your private IP in the future)
  fastcgi_index  index.php;
  fastcgi_param  SCRIPT_FILENAME  $document_root$fastcgi_script_name;
  include        fastcgi_params;
}
}

Second Application

server{
listen 80;
server_name php2.localhost; 

root   /home/www/php5.6;

location / {
  index  index.php;
}

location ~ \.php$ {
  fastcgi_pass   127.0.0.1:9004; (You can change it to your private IP in the future)
  fastcgi_index  index.php;
  fastcgi_param  SCRIPT_FILENAME  $document_root$fastcgi_script_name;
  include        fastcgi_params;
}
}

By using the above configuration, you can easily switch the different version result of PHP script, moreover, you can use NFS(if you are in a *nix environment) to mount the disk, in this case, you will no need to put files into two folders manually.

As for Fastcgi passing method, I suggest using PORT instead of Socket.

We all know Unix Socket has a better performance due to TCP port uses the whole network stack even on the same machine.

However, the time you save is very little, moreover, you may face this problem when using Socket in a high traffic time,

connect() to unix:/var/run/php5-fpm.sock failed or apr_socket_recv: Connection reset by peer (104)

In addition, TCP port can give you a faster way to manage if you put nginx and php-fpm in separated servers.

I'm using small laptop to edit this post, so codes are not pretty but I tried....

EDITED

Remember to modify your /etc/hosts to match your hostname (server_name in nginx.conf)

Installed PHP7.4 on Ubuntu 18.04, running Nginx only no Apache, the conf file is in /etc/nginx/conf.d not /etc/nginx/sites-available/default in fact the sites-available folder is not even there.

And the line “fastcgi_pass unix:/var/run/php/php7.3-fpm.sock;” is not in the conf file, I have tried to add it but get the error " “fastcgi_pass” directive is not allowed here in" when running nginx -t

Any ideas?

Thanks