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In Joomla! there are three possible applications: Administrator, Site, and Installation. Each application extends the abstract base class JApplication with objects JAdministrator, JSite, and JInstallation respectively.
JApplication | |---> JAdministrator | |---> JSite | |---> JInstallation
The Joomla! 1.5 framework uses the global variable $mainframe to reference the Application object. $mainframe is an object of type JAdministrator, JSite or JInstallation depending on the application currently running.
This provides an easy way for extensions to reference global application properties and methods without having to be concerned with which Application object is currently being referenced. When a component is operating in the backend, $mainframe will be pointing to the JAdministrator Application object. When a component is operating in the frontend, $mainframe will be pointing to the JSite Application object. During installation, $mainframe will be pointing to the JInstallation Application object.
JAdministrator, JSite and JInstallation are not documented or found on api.joomla.org because they are extended classes of JApplication and not the framework. The JApplication class is documented on api.joomla.org since it is part of the core. The JSite and JAdministrator classes extend JApplication with methods and properties specific to their frontend or backend requirements.
NOTE: The use of the global variable $mainframe has been deprecated as of 1.5 and has been removed in 1.6.
$app = JFactory::getApplication();
Please note that the variable $app is totally arbitrary. You could use $mainframe if it reduces the number of changes required throughout your code, for example:
$mainframe = JFactory::getApplication();