This is a simple convenience script to switch between PHP 5 and PHP 7 on UBUNTU both for the CLI and APACHE.

swapPHP.bash

#!/bin/bash

###############################################################################
#   This is a simple convenience script to switch between PHP 5 and PHP 7 on
#   UBUNTU both for the CLI and APACHE.
#
#   Just call the script with the first argument set to `apache` or `cli`. The
#   script will try to dectect if you are currently using PHP 5.6 or PHP 7.1
#   and switch to the other version.
#
#   This script assumes you installed PHP5.6 and PHP7.1 in parallel using the
#   method describe in
#   https://askubuntu.com/questions/761713/how-can-i-downgrade-from-php-7-to-php-5-6-on-ubuntu-16-04
#
#   DO NOT USE IN PRODUCTION ENVIRONMENT.
#
#   USE AT YOUR OWN RISK. IF IT BLOWS IN YOUR FACE, IT AIN'T MY FAULT!
#
###############################################################################

# PHP7 MINOR version to use
PHPSEVENMINOR=1

# Make sure the user has root
if [ "$EUID" -ne 0 ]; then 
  echo "Please run as root" >&2
  exit 10;
fi

# The use wants to update the PHP version for APACHE.
if [ $1 = "apache" ]; then

  apacheModPath=/etc/apache2/mods-enabled

  if [ -f $apacheModPath/php5.6.conf ]; then

    a2dismod php5.6 >/dev/null
    a2enmod php7.$PHPSEVENMINOR >/dev/null
    service apache2 restart >/dev/null

    echo APACHE now using PHP 7
  elif [ -f $apacheModPath/php7.$PHPSEVENMINOR.conf ]; then

    a2dismod php7.$PHPSEVENMINOR >/dev/null
    a2enmod php5.6 >/dev/null
    service apache2 restart >/dev/null

    echo APACHE now using PHP 5
  else
    echo "Unknown version of PHP"  >&2
    exit 10
  fi

  exit

fi

# The user wants to update the PHP version for the CLI
if [ $1 = "cli" ]; then
  PHPVERSION=$(php -v|grep --only-matching --perl-regexp "((5\.6)|(7\.$PHPSEVENMINOR))"|head -1)
  if [ $PHPVERSION = "5.6" ]; then
    update-alternatives --set php /usr/bin/php7.$PHPSEVENMINOR >/dev/null

    echo CLI now using PHP 7

  elif [ $PHPVERSION = "7.$PHPSEVENMINOR" ]; then
    update-alternatives --set php /usr/bin/php5.6 >/dev/null

    echo CLI now using PHP 5
  else
    echo "Unknown version of PHP" >&2
    exit 10
  fi

  exit
fi

# The user didn't provide the first parameter or it was non-sense.
echo "First parameter must be provided and must be either \`apache\` or \`cli\`" >&2
exit 10