There is an error when private constant is default value in method parameter.

Minimal test file:

<?php

use PHPUnit\Framework\TestCase;

class SomeClass
{
    private const CLASS_CONSTANT = 42;
    public function someMethod(int $value = self::CLASS_CONSTANT)
    {
        return $value;
    }
}

class SomeClassTest extends TestCase
{
    public function testSomeMethod()
    {
        $stub = $this->createMock(SomeClass::class);
        $stub->method('someMethod')->willReturn(42);
        $this->assertSame(42, $stub->someMethod());
    }
}

Example execution via phpunit.phar:

root@48a85a9a7259:/# phpunit /usr/src/SomeClassTest.php
PHPUnit 8.3.4 by Sebastian Bergmann and contributors.

E                                                                   1 / 1 (100%)

Time: 67 ms, Memory: 10.00 MB

There was 1 error:

1) SomeClassTest::testSomeMethod
Error: Undefined class constant 'self::CLASS_CONSTANT'

/usr/src/SomeClassTest.php:20

ERRORS!
Tests: 1, Assertions: 0, Errors: 1.

I dig a little deeper and find the issue. There are some changes in PHPUnit with constants handle when mock method is creating - 83722bc. And here is the reason - sebastianbergmann/phpunit-mock-objects#419.
That was a PHP bug and I found it in PHP changelog:
Fixed bug #76717 (var_export() does not create a parsable value for PHP_INT_MIN)
https://bugs.php.net/bug.php?id=76717
This fix is released in versions 7.2.17 and 7.3.4