CODE EXAMPLE FOR PHP

root composer.json requires php ^7.1.3 but your php version (8.0.1) does not satisfy that requirement.

for windows 
download php version you require
https://www.php.net/releases/index.php
then in variable environment of windows, edit PATH key, find php and update path to your new php version(after extract)

for ubuntu/linux
sudo apt-get install software-properties-common
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt-get install php7.1

then if in future you want to switch between your php versions, 
sudo update-alternatives --config php