At what point is session data available?

I have the following so far opening a session (php),

session_start() ;

foreach($_SESSION as $key => $value){
    $alert .= '<br/>' . '['.$key.']='.$value ;
}

$alert .= '<br/>$_SERVER[MY_SERVER...]='.$_SERVER['MY_SERVER_GENERATED'] ;

if($_SESSION['MY_SERVER_GENERATED'] !== true ||
    $_SERVER['REMOTE_ADDR'] !== $_SESSION['PREV_REMOTEADDR'] ||
    $_SERVER['HTTP_USER_AGENT'] !== $_SESSION['PREV_USERAGENT']){
        $alert .= 'session destroyed' ;
        session_destroy() ;
}

if(isset($_SESSION['CREATED']) && (strtotime('now') - $_SESSION['CREATED']) > 300){//Delete expired sessions - seconds
    $alert .= '<div class="error_box">Your session has expired!</div>' ;
    $_SESSION = array() ;
}

session_regenerate_id();
$_SESSION['CREATED'] = strtotime('now') ;
$_SESSION['MY_SERVER_GENERATED'] = true ;
$_SESSION['PREV_USERAGENT'] = $_SERVER['HTTP_USER_AGENT'] ;
$_SESSION['PREV_REMOTEADDR'] = $_SERVER['REMOTE_ADDR'] ;

echo $alert ;

The problem is the session is being dest开发者_如何转开发royed everytime. $_SESSION is simply empty. At what point is it available to run checks like above?

Oops forgot to specifiy that I have verified the session is holding data but not until the page has been started. So the session is being destroyed because the $_SESSION data is all null and so the conditions are evaluating to false. As an example $alert does not register as having anything stored in it.

Your code seems to have a catch 22:

session_start() ;


if(!$_SESSION['MY_SERVER_GENERATED']){ <------- this will always be true 
                                                for a fresh session, so
    session_destroy() ;  <--------------------- this will always be executed

your session data never has a chance to populate.