Sessions are a simple way to store data for individual users against a unique session ID. This can be used to persist state information between page requests. Session IDs are normally sent to the browser via session cookies and the ID is used to retrieve existing session data. The absence of a ID or session cookie let's PHP know to create a new session, and generate a new session ID.
Sessions follow a simple workflow. When a session is started, PHP will either retrieve an existing session using the ID passed (usually from a session cookie) or if no session is passed it will create a new session. PHP will populate the $_SESSION superglobal with any session data after the session has started. When PHP shuts down, it will automatically take the contents of the $_SESSION superglobal, serialize it, and send it for storage using the session save handler.
By default, PHP uses the internal files save handler which
is set by session.save_handler.
This saves session data on the server at the location specified by the
session.save_path configuration directive.
Sessions can be started manually using the session_start() function,
and if the session.auto_start directive is set
to 1, a session will automatically start the moment PHP sends any output
to the output buffer.
Sessions normally shutdown automatically when PHP is finished executing a script, but can be manually shutdown using the session_write_close() function.
Example #1 Registering a variable with $_SESSION.
<?php
session_start();
if (!isset($_SESSION['count'])) {
$_SESSION['count'] = 0;
} else {
$_SESSION['count']++;
}
?>
Example #2 Unregistering a variable with $_SESSION.
<?php
session_start();
unset($_SESSION['count']);
?>
Do NOT unset the whole $_SESSION with unset($_SESSION) as this will disable the registering of session variables through the $_SESSION superglobal.
You can't use references in session variables as there is no feasible way to restore a reference to another variable.