nixos composer issues

➜  php_project composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for phpstan/phpstan ^0.10.2 -> satisfiable by phpstan/phpstan[0.10.2].
    - phpstan/phpstan 0.10.2 requires php ~7.1 -> your PHP version (5.6.36) does not satisfy that requirement.
  Problem 2
    - phpunit/phpunit 7.2.7 requires php ^7.1 -> your PHP version (5.6.36) does not satisfy that requirement.
    - phpunit/phpunit 7.2.6 requires php ^7.1 -> your PHP version (5.6.36) does not satisfy that requirement.
    - phpunit/phpunit 7.2.5 requires php ^7.1 -> your PHP version (5.6.36) does not satisfy that requirement.
    - phpunit/phpunit 7.2.4 requires php ^7.1 -> your PHP version (5.6.36) does not satisfy that requirement.
    - phpunit/phpunit 7.2.3 requires php ^7.1 -> your PHP version (5.6.36) does not satisfy that requirement.
    - phpunit/phpunit 7.2.2 requires php ^7.1 -> your PHP version (5.6.36) does not satisfy that requirement.
    - phpunit/phpunit 7.2.1 requires php ^7.1 -> your PHP version (5.6.36) does not satisfy that requirement.
    - phpunit/phpunit 7.2.0 requires php ^7.1 -> your PHP version (5.6.36) does not satisfy that requirement.
    - Installation request for phpunit/phpunit ^7.2 -> satisfiable by phpunit/phpunit[7.2.0, 7.2.1, 7.2.2, 7.2.3, 7.2.4, 7.2.5, 7.2.6, 7.2.7].

➜  php_project php -v
PHP 7.2.7 (cli) (built: Jun 19 2018 14:40:28) ( ZTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies