This seems to happen only on my system. Nobody else could reproduce it. If the error's message is too large (about 65k bytes in my case) nothing gets printed on screen. I'm using PHP 7.1.3 on Windows 7, with the default php.ini (memory limit set to 8gb) and the default PHPUnit 6.0.13 configuration. The error does not appear in both prompt and powershell.

<?php

use PHPUnit\Framework\Constraint\Constraint;
use PHPUnit\Framework\TestCase;

class MyConstraint extends Constraint
{
    protected $expected;

    function __construct($expected){
        parent::__construct();
        $this->expected = $expected;
    }

    protected function matches($actual){
        return false;
    }

    function failureDescription($other): string{
        return "desc";
    }

    function toString(){
        return "description";
    }

    function additionalFailureDescription($other): string{
        return str_repeat("x", 100000);
        // If set to a smaller dump, error will appear
        // some people I asked to try could dump one million
        // bytes without problems, while I can't print more
        // than about 50k
    }
}

class SomeTest extends TestCase
{
    function testBigDump(){
        TestCase::assertThat("irrelevant", new MyConstraint("irrelevant"));
    }
}

?>

And this is what I get on screen:

PHPUnit 6.0.13 by Sebastian Bergmann and contributors.

Runtime: PHPDBG 7.1.3 Configuration: ..............

F 1 / 1 (100%)

Time: 361 ms, Memory: 6.00MB

There was 1 failure:

1) SomeTest::testBigDump

                             <------- Notice no error description here

FAILURES! Tests: 1, Assertions: 1, Failures: 1.

Do you have any idea what could cause this? Thank you in advance.