composer.jsonがないときは

echo {} > composer.json

composer config minimum-stability dev

エラー内容

composer require amphp/http-client:v5.x-dev
./composer.json has been updated
Running composer update amphp/http-client
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires amphp/http-client v5.x-dev -> satisfiable by amphp/http-client[v5.x-dev].
    - amphp/http-client v5.x-dev requires php >=8.1 -> your php version (7.2.34) does not satisfy that requirement.


Installation failed, reverting ./composer.json to its original content.