phpunit 5.6.4 doesn't work in php 5.6 for doctrine's dependency via composer

Question

I have a problem composer install dependencies. We use php which is version 5.6.24. When installing phpunit 5.6.4, we install doctrine needs php^7. This is our partial composer.lock after require and install phpunit.

"packages-dev": [
    {
        "name": "doctrine/instantiator",
        "version": "1.1.0",
        "source": {
            "type": "git",
            "url": "https://github.com/doctrine/instantiator.git",
            "reference": "185b8868aa9bf7159f5f953ed5afb2d7fcdc3bda"
        },
        "dist": {
            "type": "zip",
            "url": "https://api.github.com/repos/doctrine/instantiator/zipball/185b8868aa9bf7159f5f953ed5afb2d7fcdc3bda",
            "reference": "185b8868aa9bf7159f5f953ed5afb2d7fcdc3bda",
            "shasum": ""
        },
        "require": {
            "php": "^7.1"
        },

Why does it cause and How do we fix this dependency from php 7 to 5? As I thought, It is possible of using phpunit 5.6.4 in php 5 as https://packagist.org/packages/phpunit/phpunit#5.6.4 show. (We use cakephp 2.10.3 depending on sebastian/exporter 1.2. So we can use only under 5.6.4 phpunit version.)

and we use docker ,so we install library following command .

docker run --rm -it -v $(pwd):/app composer:1.4.1 require --dev phpunit/phpunit:"5.6.4" --ignore-platform-reqs --update-with-dependencies 

Answer 1

This problem is caused by incompatible of doctrine version (see http://doctrine-project.org/2017/07/25/php-7.1-requirement-and-composer.html) and phpdocumentor/reflection-docblock and as it show. I can override invalid versions to use command composer

require (or --dev) package:$correct_version

Of course , as well as docker environment.

Composer file can have ambiguous dependencies' match setting like ＾、〜 operator, therefore dependencies will be broken if library installed to be required by dependencies change requirements which has updated incompatible before. So we need implicitly definitions of their correct version in composer.json. Thanks to @TonyChiboucas's comment add your own requirement definition . It becomes a clue to resolve my problem.