Sessions
Session support in PHP consists of a way to preserve certain data across subsequent accesses. This enables you to build more customized applications and increase the appeal of your web site. All information is in the Session reference section.
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A session is a way to store information (in variables) to be used across multiple pages.
Session variables solve a very big problem by storing user information to be used across multiple pages (e.g. username, favorite color, etc). By default, session variables last until the user closes the browser.
Session variables hold information about one single user, and are available to all pages in one application.
SESSION_START();
it is used to start the session.