$ php --version

PHP 5.6.5 (cli) (built: Feb 12 2015 01:41:10)

Copyright (c) 1997-2014 The PHP Group

Zend Engine v2.6.0, Copyright (c) 1998-2014 Zend Technologies

    with Zend OPcache v7.0.4-dev, Copyright (c) 1999-2014, by Zend Technologies

    with Xdebug v2.2.7, Copyright (c) 2002-2015, by Derick Rethans

$ composer --version

Composer version 1.3.2 2017-01-27 18:23:41

18.43s$ 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

    - phpunit/phpunit 6.0.7 requires php ^7.0 -> your PHP version (5.6.5) does not satisfy that requirement.

    - phpunit/phpunit 6.0.6 requires php ^7.0 -> your PHP version (5.6.5) does not satisfy that requirement.

    - phpunit/phpunit 6.0.5 requires php ^7.0 -> your PHP version (5.6.5) does not satisfy that requirement.

    - phpunit/phpunit 6.0.4 requires php ^7.0 -> your PHP version (5.6.5) does not satisfy that requirement.

    - phpunit/phpunit 6.0.3 requires php ^7.0 -> your PHP version (5.6.5) does not satisfy that requirement.

    - phpunit/phpunit 6.0.2 requires php ^7.0 -> your PHP version (5.6.5) does not satisfy that requirement.

    - phpunit/phpunit 6.0.1 requires php ^7.0 -> your PHP version (5.6.5) does not satisfy that requirement.

    - phpunit/phpunit 6.0.0 requires php ^7.0 -> your PHP version (5.6.5) does not satisfy that requirement.

    - Installation request for phpunit/phpunit ^6.0 -> satisfiable by phpunit/phpunit[6.0.0, 6.0.1, 6.0.2, 6.0.3, 6.0.4, 6.0.5, 6.0.6, 6.0.7].