? Unit test ?

ConfigurationAdapterInterface.php

<?php

namespace Qbil\CommonBundle\Services;

use Qbil\CommonBundle\Exception\LoadingConfigurationFailedException;

interface ConfigurationAdapterInterface
{
    /**
     * @throws LoadingConfigurationFailedException
     */
    public function getConfiguration(): array;
}

LegacyConfigurationParser.php

<?php

namespace Qbil\LegacyBridgeBundle\Services;

use Qbil\CommonBundle\Services\ConfigurationAdapterInterface;
use Symfony\Component\PropertyAccess\PropertyAccess;

class LegacyConfigurationParser implements LegacyConfigurationParserInterface
{
    public static $configuration;

    public function __construct(ConfigurationAdapterInterface $configurationAdapter)
    {        
        self::$configuration = $configurationAdapter->getConfiguration();
    }

    public function isLoaded(): bool
    {
        return null !== self::$configuration;
    }

    public function getSetting($path)
    {
        $accessor = PropertyAccess::createPropertyAccessor();
        try {
            $setting = $accessor->getValue(self::$configuration, $path);
        } catch (\Exception $e) {
            $setting = null;
        }

        return $setting;
    }
}

LegacyConfigurationParserInterface.php

<?php

namespace Qbil\LegacyBridgeBundle\Services;

interface LegacyConfigurationParserInterface
{
    public function isLoaded(): bool;

    public function getSetting($path);
}

LegacyConfigurationParserTest.php

<?php

namespace Qbil\Tests\LegacyBridgeBundle\Services;

use PHPUnit\Framework\TestCase;
use Qbil\CommonBundle\Services\ConfigurationAdapterInterface;
use Qbil\LegacyBridgeBundle\Services\LegacyConfigurationParser;

class LegacyConfigurationParserTest extends TestCase
{
    private $legacyConfigParser;

    protected function setUp(): void
    {
        $configurationAdapter = $this->createMock(ConfigurationAdapterInterface::class);

        $configurationAdapter
            ->expects($this->once())
            ->method('getConfiguration')
            ->willReturn(['foo' => 'bar']);

        $this->legacyConfigParser = new LegacyConfigurationParser($configurationAdapter);
    }

    public function testIsLoaded()
    {
        $this->assertTrue($this->legacyConfigParser->isLoaded());
    }

    public function testGetSetting()
    {
        $this->assertSame('bar', $this->legacyConfigParser->getSetting('[foo]'));
        $this->assertNull($this->legacyConfigParser->getSetting('[baz]'));
    }
}

PS: https://gist.github.com/faizanakram99/a14e9189ccf4bf7bb87bb90fa4d5d944#file-legacyconfigurationparser-php-L14 is for backward compatibility, a lot of legacy code depends upon global $configuration variable

it can be still improved though .. well lets leave as it is for now