Use constant instead of magic string.

src/AppFactory.php

@@ -8,6 +8,7 @@ use App\Console\Application;
 use App\Enums\SupportedLocales;
 use App\Http\Factory\ResponseFactory;
 use App\Http\Factory\ServerRequestFactory;
+use App\Utilities\Logger as AppLogger;
 use DI;
 use Monolog\ErrorHandler;
 use Monolog\Logger;
@@ -106,7 +107,7 @@ class AppFactory
         $errorHandler = new ErrorHandler($logger);
         $errorHandler->registerFatalHandler();
 
-        Registry::addLogger($logger, 'app', true);
+        Registry::addLogger($logger, AppLogger::INSTANCE_NAME, true);
 
         return $di;
     }

src/Utilities/Logger.php

@@ -9,8 +9,10 @@ use Monolog\Registry;
 
 final class Logger
 {
+    public const INSTANCE_NAME = 'app';
+
     public static function getInstance(): MonologLogger
     {
-        return Registry::getInstance('app');
+        return Registry::getInstance(self::INSTANCE_NAME);
     }
 }