Compile Error problems with phpunit5.7.* and php5.6.

problem description

when using phpunit5.7.* and php5.6, assertions often have Compile Error error prompts,

the environmental background of the problems and what methods you have tried

PHP Framework: fuelphp
PHPunit version: 5.7.*
PHP version: 5.6
PHPunit installation: composer
commands used in PHP debugging

there are many theories on the Internet. I first reduced the version of phpunit to less than 5.7.*, but it was of no use.
it is also said that it is going to be upgraded to PHP7, but it is at odds with the company"s version, so there is no way to try this method.

related codes

class AucviewTest extends \PHPUnit\Framework\TestCase
{
    public function test_before()
    {
        // new
        $test = new Controller_Aucview();
    }
}

what result do you expect? What is the error message actually seen?

I have seen some videos of writing test code, but none of them mentioned compilation errors, and I personally doubt whether the frequent compilation errors are PHP versions.

Compile Error-Default value for parameters with a class type hint can only be NULL