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 an ID or session cookie lets 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.
If the session.auto_start directive is set
to 1, a session will automatically start on request startup.
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 $_SESSION superglobal.
You can't use references in session variables as there is no feasible way to restore a reference to another variable.
register_globals will overwrite variables in the global scope whose names are shared with session variables. Please see Using Register Globals for details.
AlexFBP (2013-01-25 04:50:22)
Regardless, if you need to set the header 'Location:' before closing the session; explicitly close the php script with "exit()" or "die()" functions. Remember that when a php script ends, the session automatically are going to be closed.
jpleveille at webgraphe dot com (2012-09-07 18:06:36)
As mentioned in the documentation, using session_write_close() shuts down the session. It is particularly useful if you want to use header('Location: SOMEURL'); to a URL within the same scope of the current script, that will load the session. Why?
When you use this header directive, the browser is requested to redirect the user to the given URL. If that URL is in the scope of the script where header() is called (let's say, same URL), the requested URL "COULD" load the session BEFORE it has actually been shut down in the previous script, and you might end up with the session from the previous request, reverting all modifications to session in the the last script.
<?php
session_start();
if (!isset($_SESSION['hello']))
{
$_SESSION['hello'] = 'world';
session_write_close();
// session is now closed, it's safe to redirect
// if not closed, $_SESSION['hello'] may not be set properly
// when loading the page again
// (in this very case, calls to this script could loop for a while)
header('Location: ' . $_SERVER['PHP_SELF']);
}
?>