Configurations are used to create new objects or initializing the existing ones. Configurations usually include a class name and a list of initial values. They may also include a list of event handlers and behaviors.

The following is an example of the database configuration −

<?php
   $config = [
      ''class'' => ''yiidbConnection'',
      ''dsn'' => ''mysql:host = localhost;dbname = helloworld'',
      ''username'' => ''vladimir'',
      ''password'' => ''12345'',
      ''charset'' => ''utf8'',
   ];
   $db = Yii::createObject($config);
?>

The Yii::createObject() method takes a configuration array and creates an object based on the class named in the configuration.

The format of a configuration −

[
   //a fully qualified class name for the object being created
   ''class'' => ''ClassName'',
   //initial values for the named property
   ''propertyName'' => ''propertyValue'',
   //specifies what handlers should be attached to the object''s events
   ''on eventName'' => $eventHandler,
   //specifies what behaviors should be attached to the object
   ''as behaviorName'' => $behaviorConfig,
]

The configuration file of a basic application template is one of the most complex −

<?php
   $params = require(__DIR__ . ''/params.php'');
   $config = [
      ''id'' => ''basic'',
      ''basePath'' => dirname(__DIR__),
      ''bootstrap'' => [''log''],
      ''components'' => [
         ''request'' => [
            // !!! insert a secret key in the following (if it is empty) - this 
               //is required by cookie validation
            ''cookieValidationKey'' => ''ymoaYrebZHa8gURuolioHGlK8fLXCKjO'',
         ],
         ''cache'' => [
            ''class'' => ''yiicachingFileCache'',
         ],
         ''user'' => [
            ''identityClass'' => ''appmodelsUser'',
            ''enableAutoLogin'' => true,
         ],
         ''errorHandler'' => [
            ''errorAction'' => ''site/error'',
         ],
         ''mailer'' => [
            ''class'' => ''yiiswiftmailerMailer'',
            // send all mails to a file by default. You have to set
            // ''useFileTransport'' to false and configure a transport
            // for the mailer to send real emails.
            ''useFileTransport'' => true,
         ],
         ''log'' => [
            ''traceLevel'' => YII_DEBUG ? 3 : 0,
            ''targets'' => [
               [
                  ''class'' => ''yiilogFileTarget'',
                  ''levels'' => [''error'', ''warning''],
               ],
            ],
         ],
         ''urlManager'' => [
            //''showScriptName'' => false,
            //''enablePrettyUrl'' => true,
            //''enableStrictParsing'' => true,
            //''suffix'' => ''/''
         ],
         ''db'' => require(__DIR__ . ''/db.php''),
      ],
      ''modules'' => [
         ''hello'' => [
            ''class'' => ''appmoduleshelloHello'',
         ],
      ],
      ''params'' => $params,
   ];
   if (YII_ENV_DEV) {
      // configuration adjustments for ''dev'' environment
      $config[''bootstrap''][] = ''debug'';
      $config[''modules''][''debug''] = [
         ''class'' => ''yiidebugModule'',
      ];
      $config[''bootstrap''][] = ''gii'';
      $config[''modules''][''gii''] = [
         ''class'' => ''yiigiiModule'',
      ];
   }
   return $config;
?>

In the above configuration file, we do not define the class name. This is because we have already defined it in the index.php file −

<?php
   //defining global constans
   defined(''YII_DEBUG'') or define(''YII_DEBUG'', true);
   defined(''YII_ENV'') or define(''YII_ENV'', ''dev'');
   //register composer autoloader
   require(__DIR__ . ''/../vendor/autoload.php'');
   //include yii files
   require(__DIR__ . ''/../vendor/yiisoft/yii2/Yii.php'');
   //load application config
   $config = require(__DIR__ . ''/../config/web.php'');
   //create, config, and process request
   (new yiiwebApplication($config))->run();
?>

Many widgets also use configurations as shown in the following code.

<?php
   NavBar::begin([
      ''brandLabel'' => ''My Company'',
      ''brandUrl'' => Yii::$app->homeUrl,
      ''options'' => [
         ''class'' => ''navbar-inverse navbar-fixed-top'',
      ],
   ]);
   echo Nav::widget([
      ''options'' => [''class'' => ''navbar-nav navbar-right''],
      ''items'' => [
         [''label'' => ''Home'', ''url'' => [''/site/index'']],
         [''label'' => ''About'', ''url'' => [''/site/about'']],
         [''label'' => ''Contact'', ''url'' => [''/site/contact'']],
         Yii::$app->user->isGuest ?
         [''label'' => ''Login'', ''url'' => [''/site/login'']] :
         [
            ''label'' => ''Logout ('' . Yii::$app->user->identity->username . '')'',
            ''url'' => [''/site/logout''],
            ''linkOptions'' => [''data-method'' => ''post'']
         ],
      ],
   ]);
   NavBar::end();
?>

When a configuration is too complex, a common practice is to create a PHP file, which returns an array. Take a look at the config/console.php configuration file −

<?php
   Yii::setAlias(''@tests'', dirname(__DIR__) . ''/tests'');

   $params = require(__DIR__ . ''/params.php'');
   $db = require(__DIR__ . ''/db.php'');

   return [
      ''id'' => ''basic-console'',
      ''basePath'' => dirname(__DIR__),
      ''bootstrap'' => [''log'', ''gii''],
      ''controllerNamespace'' => ''appcommands'',
      ''modules'' => [
         ''gii'' => ''yiigiiModule'',
      ],
      ''components'' => [
         ''cache'' => [
            ''class'' => ''yiicachingFileCache'',
         ],
         ''log'' => [
            ''targets'' => [
               [
                  ''class'' => ''yiilogFileTarget'',
                  ''levels'' => [''error'', ''warning''],
               ],
            ],
         ],
         ''db'' => $db,
      ],
      ''params'' => $params,
   ];
?>

The default configurations can be specified by calling the Yii::$container->set() method. It allows you to apply default configurations to all instances of the specified classes when they are called via the Yii::createObject() method.

For example, to customize the yiiwidgetsLinkPager class, so that all link pagers will show at most three buttons, you can use the following code.

Yii::$container->set(''yiiwidgetsLinkPager'', [
   ''maxButtonCount'' => 3,
]);