:: operator is used to access static methods that do not require object initialization.

The use of the function session_start() lets us activating a session.

The boolean function isset determines if a variable is set and is not NULL.

If we try to copy a referenced object, it will simply point back to the addressing location of the original object. To remedy the problems with copying, PHP offers an explicit means for cloning an object which is known as object cloning.

Configuration directive scopes are given bellow:
PHP_INI_PERDIR: Directive can be modified within the php.ini,httpd.conf, or .htaccess files.
PHP_INI_SYSTEM: Directive can be modified within the php.ini and httpd.conf files.
PHP_INI_USER: Directive can be modified within user scripts.
PHP_INI_ALL: Directive can be modified anywhere.