If variable is set, show $_SESSION data

If variable is set, show $_SESSION data

I have a hard time with isset() function. Exactly how do I tell php that
only if the data was assigned to my, lets say $_SESSION['msg']; variable,
only then activate it and use it... So far I am getting a lot of notices:
undefined index and this code doesn't do it also:
if (isset($_SESSION['monster']) &
isset($_SESSION['spol']) &
isset($_SESSION['combat']) &
isset($_SESSION['turns']) &
isset($_SESSION['zmaga']) &
isset($_SESSION['zguba']) &
isset($_SESSION['cekini']) &
isset($_SESSION['post']) )
{
$monster = $_SESSION['monster'];
$spol = $_SESSION['spol'];
$combat = $_SESSION['combat'];
$turns = $_SESSION['turns'];
$zmaga = $_SESSION['zmaga'];
$zguba = $_SESSION['zguba'];
$cekini = $_SESSION['cekini'];
$_POST = $_SESSION['post'];
}
Data was supposed to be asigned to it from a redirecting previous site but
that site doesn't always activate so I need an alternative. That's why I
am asking this.
EDIT with full code:
session_start();
include 'razno.php';
include 'save.php';
include 'stats.php';
$igralec_ime = $_SESSION['username'];
$monster = prikazi_borba($igralec_ime);
$poskodbe = prikazi_stat('curhp', $igralec_ime);
if (prikazi_stat('curhp', $igralec_ime) == 0)
{
$moznost = 'Tvoje zdravje je resno ogroženo, vrni se domov!';
}
else if (isset($_SESSION['monster']) &&
isset($_SESSION['spol']) &&
isset($_SESSION['combat']) &&
isset($_SESSION['turns']) &&
isset($_SESSION['zmaga']) &&
isset($_SESSION['zguba']) &&
isset($_SESSION['cekini']) &&
isset($_SESSION['post']) )
{
$monster = $_SESSION['monster'];
$spol = $_SESSION['spol'];
$combat = $_SESSION['combat'];
$turns = $_SESSION['turns'];
$zmaga = $_SESSION['zmaga'];
$zguba = $_SESSION['zguba'];
$cekini = $_SESSION['cekini'];
$_POST = $_SESSION['post'];
}
update_save($igralec_ime, 'gozd', $monster);
include '../html/gozd.html';
?>