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• Composer 2.x is not compatible with the dependencies described in the composer.json file.
• But, the default composer installer installs the latest version 2.x.
• we need to download Composer 1.x via the following URL: https://getcomposer.org/composer-1.phar

- php -r "readfile('https://getcomposer.org/installer');" | sudo php -- --install-dir=/usr/local/bin --filename=composer
+ cd /usr/local/bin
+ wget --no-verbose  https://getcomposer.org/composer-1.phar
+ mv composer-1.phar composer
+ chmod +x composer

phpro/grumphp 0.15.2

phpro/grumphp is used to check commits. Version grumphp 0.15.2 is the latest version compatible with php 7.0, but is not compatible with composer v2.

see: https://packagist.org/packages/phpro/grumphp#v0.15.2

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires phpro/grumphp ^0.15.2 
      -> satisfiable by phpro/grumphp[v0.15.2].
    - phpro/grumphp v0.15.2 requires composer-plugin-api ~1.0 
      -> found composer-plugin-api[2.0.0] but it does not match the constraint.

You are using Composer 2, which some of your plugins seem to be incompatible with. 
Make sure you update your plugins or report a plugin-issue 
to ask them to support Composer 2.