Uruchom kompozytor z inną wersją PHP
php7.3 /usr/local/bin/composer install
Delightful Dolphin
php7.3 /usr/local/bin/composer install#/usr/bin/php7.1 /usr/local/bin/composer update
#or edit your composer.json file and specify the version you want
"config": {
"platform": {
"php": "7.1.3"
}
}