PHPUnit 6.1.x throws array_merge() error when my test class uses its own constructor method

PHPUnit uses the constructor for initialization of the base TestCase

You can see the constructor method here: https://github.com/sebastianbergmann/phpunit/blob/6.1.3/src/Framework/TestCase.php#L328

public function __construct($name = null, array $data = [], $dataName = '')

You shouldn't use the constructor, because it's used by phpunit and any change to the signature etc can break things.

You can use the special setUp and setUpBeforeClass methods which phpunit will call for you.

use PHPUnit\Framework\TestCase;

class XTest extends TestCase
{
    function static setUpBeforeClass()
    { 
       // Called once just like normal constructor
       // You can create database connections here etc
    }

    function setUp()
    {
      //Initialize the test case
      //Called for every defined test
    }

    function testX()
    {
        $this->assertTrue(true);
    }

    // Clean up the test case, called for every defined test
    public function tearDown() { }

    // Clean up the whole test class
    public static function tearDownAfterClass() { }
}

The docs: https://phpunit.de/manual/current/en/fixtures.html

Note that the setUp gets called for every specified test in the class.

For a single initialization you can use setUpBeforeClass.

And another tip: run your phpunit with the -v flag to display stack traces ;)

As Sander Visser's answer correctly pointed, the parent constructor might have additional parameters, etc, and generally you'd want to use setUpBeforeClass or setUp, but, if you know what you're doing, you can call parent::__construct(); in the constructor of your Test class:

public function __construct() {
    parent::__construct();
    // Your construct here
}

Edit 2019

In Codeception, this can also be caused by an invalid YML indentation on your suite file.