You can try setting the path to the correct php version on the terminal command line:

set PATH="/usr/bin/php5.6/bin:$PATH"

Modify the path to match your own path to PHP 5.6 (ie. if it was installed first, it might be something like "/usr/bin/php").

The "$PATH" at the end appends the current path value, so don't forget to use it.

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

Please see: Source

From PHP 7.1 => PHP 5.6

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

From PHP 5.6 => PHP 7.1

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

You can change the versions as you want.

You can check this tutorial

Regards!!

This explanation is based on Ubuntu 16.04 but is expected to work for other versions too

Most answers here manipulate the php-version by using the command

sudo update-alternatives --set ...

While the command is quite useful it's never explained what it does exactly. Here comes the explanation, including backup and some options:

Get the Information

• The command update-alternatives is displaying or changing symlinks which reside in an alternative direction which is in Ubuntu usually defined as /etc/alternatives but which could be changed too.
• a full list of options related to update-alternatives can be shown with the command update-alternatives --help, a deeper explanation can be shown with man update-alternatives.
• As the command update-alternatives is primary changing symlinks in a special folder, the content of that folder can be shown with common commands too, following all items starting with php are shown:

 $ ls -al /etc/alternatives/php*
lrwxrwxrwx 1 root root 15 Jan 19 02:58 /etc/alternatives/php -> /usr/bin/php7.2
lrwxrwxrwx 1 root root 31 Jan 19 02:58 /etc/alternatives/php.1.gz -> /usr/share/man/man1/php7.2.1.gz
lrwxrwxrwx 1 root root 19 Jan 19 03:00 /etc/alternatives/php-cgi -> /usr/bin/php-cgi7.2
lrwxrwxrwx 1 root root 35 Jan 19 03:00 /etc/alternatives/php-cgi.1.gz -> /usr/share/man/man1/php-cgi7.2.1.gz
lrwxrwxrwx 1 root root 23 Jan 19 03:00 /etc/alternatives/php-cgi-bin -> /usr/lib/cgi-bin/php7.2

• to display the items with the command update-alternatives use this command:

$ update-alternatives --list php
/usr/bin/php7.0
/usr/bin/php7.2

• to show more details you can execute this:

$ update-alternatives --display php
php - auto mode
link best version is /usr/bin/php7.2
link currently points to /usr/bin/php7.2
link php is /usr/bin/php
slave php.1.gz is /usr/share/man/man1/php.1.gz
/usr/bin/php7.0 - priority 70
slave php.1.gz: /usr/share/man/man1/php7.0.1.gz
/usr/bin/php7.2 - priority 72
slave php.1.gz: /usr/share/man/man1/php7.2.1.gz

Finally we still want to know which php-versions we can link. The following command shows a list of the currently available php-versions in /usr/bin/:

$ ls -al /usr/bin/php*
lrwxrwxrwx 1 root root      21 Jan  1 19:47 /usr/bin/php -> /etc/alternatives/php
-rwxr-xr-x 1 root root 4385840 Apr  5 18:13 /usr/bin/php7.0
-rwxr-xr-x 1 root root 4875488 Apr  5 18:10 /usr/bin/php7.2
lrwxrwxrwx 1 root root      25 Jan  1 19:47 /usr/bin/php-cgi -> /etc/alternatives/php-cgi
-rwxr-xr-x 1 root root 4279672 Apr  5 18:13 /usr/bin/php-cgi7.0
-rwxr-xr-x 1 root root 4769272 Apr  5 18:10 /usr/bin/php-cgi7.2
-rwxr-xr-x 1 root root     663 Feb 29  2016 /usr/bin/phpunit

Backup the information

• To make an informational backup of these symlinks just save the list in a file and save it on your desktop with this command (adjust the filename alternatives-php.txt to your needs):

$ echo "command: update-alternatives --list php" > ~/Desktop/alternatives-php.txt
$ update-alternatives --display php >> ~/Desktop/alternatives-php.txt
$ echo "command: update-alternatives --display php" >> ~/Desktop/alternatives-php.txt
$ update-alternatives --display php >> ~/Desktop/alternatives-php.txt

if you like you still can add the result of the command ls like shown above:

$ echo "command: ls -al /etc/alternatives/php*" >> ~/Desktop/alternatives-php.txt
$ ls -al /etc/alternatives/php* >> ~/Desktop/alternatives-php.txt

... and the available PHP-versions:

$ echo "command: ls -al /usr/bin/php*" >> ~/Desktop/alternatives-php.txt
$ ls -al /usr/bin/php* >> ~/Desktop/alternatives-php.txt

Change the PHP-version for commandline

• If we filter first the man-documents the list in /etc/alternatives/php*/ includes 3 lines:

 $ ls -al /etc/alternatives/php*
lrwxrwxrwx 1 root root 15 Jan 19 02:58 /etc/alternatives/php -> /usr/bin/php7.2
lrwxrwxrwx 1 root root 19 Jan 19 03:00 /etc/alternatives/php-cgi -> /usr/bin/php-cgi7.2
lrwxrwxrwx 1 root root 23 Jan 19 03:00 /etc/alternatives/php-cgi-bin -> /usr/lib/cgi-bin/php7.2

• While on the one hand it might be useful to have consistent links for the whole system on the other hand the fact that several php-versions exist already on the system implies that it's used for development and php for cli, web and cgi could be configured differently.
  Which symlink has to be changed?
  PHP for web is usually not configured by usage of the symlinks in /etc/alternatives and cli(commandline) is usually not using the cgi-version or cgi-configuration. So it can be assumed that changing only the symlink for php will work for commandline, change of php-cgi and php-cgi-bin is probably not required. Usually the most recent version is linked by default, so the command below is using an older version taken from the list in /usr/bin/:

$ sudo update-alternatives --set php /usr/bin/php7.0
update-alternatives: using /usr/bin/php7.0 to provide /usr/bin/php (php) in manual mode

Control

Checking the change:

$: php -v
PHP 7.0.29-1+ubuntu16.04.1+deb.sury.org+1 (cli) (built: Apr  5 2018 08:34:50) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.0.0, Copyright (c) 1998-2017 Zend Technologies
with Zend OPcache v7.0.29-1+ubuntu16.04.1+deb.sury.org+1, Copyright (c) 1999-2017, by Zend Technologies

The change was accepted and the used version is an older one now.

• Now we check the version the man-page is written for:

$: man php
... [SCROLL DOWN]
VERSION INFORMATION
This manpage describes php, version 7.0.29-1+ubuntu16.04.1+deb.sury.org+1.
COPYRIGHT
...

This means the PHP-version is consistent with the man-page and the command man php returns the right descriptions for the current php-version and no manual adjustments concerning the man-page have been required.

Here is more interactive way:

update-alternatives --config php

You'll see the list of all available versions and select one of them

check what you have before changing

 php -v

change everything to php 7.2

sudo update-alternatives --set php /usr/bin/php7.2
sudo update-alternatives --set phar /usr/bin/phar7.2
sudo update-alternatives --set phar.phar /usr/bin/phar.phar7.2
sudo update-alternatives --set phpize /usr/bin/phpize7.2
sudo update-alternatives --set php-config /usr/bin/php-config7.2

Plesk

Just to say, if your server is managed using Plesk (like mine is at the time of searching through this question) then you may not have later PHP versions in /usr/bin

In this case, your PHP versions will be located in /opt/plesk/php so you can instead use the following command as a super user (run su first if you're not a super user) to set your PHP CLI version (to 7.3 in this example):

echo "alias php='/opt/plesk/php/7.3/bin/php'" >> ~/.bashrc

Log out and log back in and.... voila!

Notes

1. You must be have super user privileges to run this
2. If you want to set the CLI to a different version then simply replace the 7.3 in the command to whichever version you would like (in the OP's case this would be 5.6
3. If you want to check which versions Plesk has available for you to use, simply run: ls /opt/plesk/php

Source: Plesk Support