Here are the following core components of Yii, such as:
db:- database connection.
assetManager : manage the publishing of private asset files
authManager : manage role-based access control
cache : manage to cache functionality
clientScript : manage javascript and CSS
coreMessages : provides translated core messages
errorHandler : manage errors handling.
themeManager : Manage themes
urlManager : URL parsing and creation functionality
statePersister : a mechanism for persisting global state
session : Session management
securityManager : Security Management
