Creating New Session
==========================
<?php 
session_start();
/*session is started if you don't write this line can't use $_Session  global variable*/
$_SESSION["newsession"]=$value;
?>
Getting Session
==========================
<?php 
session_start();
/*session is started if you don't write this line can't use $_Session  global variable*/
$_SESSION["newsession"]=$value;
/*session created*/
echo $_SESSION["newsession"];
/*session was getting*/
?>
Updating Session
==========================
<?php 
session_start();
/*session is started if you don't write this line can't use $_Session  global variable*/
$_SESSION["newsession"]=$value;
/*it is my new session*/
$_SESSION["newsession"]=$updatedvalue;
/*session updated*/
?>
Deleting Session
==========================
<?php 
session_start();
/*session is started if you don't write this line can't use $_Session  global variable*/
$_SESSION["newsession"]=$value;
unset($_SESSION["newsession"]);
/*session deleted. if you try using this you've got an error*/
?>

Please note that if you have register_globals to On, global variables associated to $_SESSION variables are references, so this may lead to some weird situations.

<?php

session_start();

$_SESSION['test'] = 42;
$test = 43;
echo $_SESSION['test'];

?>

Load the page, OK it displays 42, reload the page... it displays 43.

The solution is to do this after each time you do a session_start() :

<?php

if (ini_get('register_globals'))
{
    foreach ($_SESSION as $key=>$value)
    {
        if (isset($GLOBALS[$key]))
            unset($GLOBALS[$key]);
    }
}

?>