session_start();
It Start new session or resume existing session or the current one based on the session identifier passed via any request like $_get and $_post. When a session start php will call the open and read the session handlers.User can used name of any session from the function session_name(). After creating a session user must unset the session by using void session_unset(void) function, it will free all the session variables that is currently registered

session_start();
$_SESSION['name']='NULL'
$_SESSION['password']='NULL'

session_destroy();
It is used for deleting the whole session.

session_unset();
It will deletes only the variables from session but session still exists. It will just truncate the variable.