<?php declare(strict_types=1);
/*
 * This file is part of PHPUnit.
 *
 * (c) Sebastian Bergmann <sebastian@phpunit.de>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */
namespace PHPUnit\TextUI\XmlConfiguration;

use DOMDocument;
use DOMElement;

/**
 * @internal This class is not covered by the backward compatibility promise for PHPUnit
 */
final class IntroduceCoverageElement implements Migration
{
    /**
     * @var DOMDocument
     */
    private $document;

    /**
     * @var DOMElement
     */
    private $coverage;

    public function migrate(DOMDocument $document): void
    {
        $this->document = $document;
        $this->coverage = $document->createElement('coverage');

        $this->document->documentElement->insertBefore(
            $this->coverage,
            $this->document->documentElement->firstChild
        );
    }
}

<?php declare(strict_types=1);
/*
 * This file is part of PHPUnit.
 *
 * (c) Sebastian Bergmann <sebastian@phpunit.de>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */
namespace PHPUnit\TextUI\XmlConfiguration;

use DOMDocument;
use DOMElement;

/**
 * @internal This class is not covered by the backward compatibility promise for PHPUnit
 */
final class IntroduceCoverageElement implements Migration
{
    public function migrate(DOMDocument $document): void
    {
        $coverage = $document->createElement('coverage');

        $document->documentElement->insertBefore(
            $coverage,
            $document->documentElement->firstChild
        );
    }
}

Use local variables instead of properties
Use local variables instead of properties
