Fatal error: Composer detected issues in your platform: Your Composer dependencies require a PHP version ">= 7.3.0". You are running 7.1.4. in code example

Example 1: Composer detected issues in your platform: Your Composer dependencies require a PHP version ">= 7.3.0".

composer install --ignore-platform-reqs

Example 2: Composer detected issues in your platform: Your Composer dependencies require a PHP version ">= 7.3.0".

composer install --ignore-platform-reqs

or 
 #add to composer.json > config
"platform-check": false,

composer autoload-dump