// Your array index has no value or is not referencing anything,
// The easiest way to overcome this is to simply check whether
// it has been defined
if (isset($arr[$i])) {
// Do something
}$_SESSION['yy'] = isset($_POST['yy']) ? $_POST['yy'] : ""; // This is on line 11
$_SESSION['mm'] = isset($_POST['mm']) ? $_POST['mm'] : ""; // This is on line 12
$_SESSION['dd'] = isset($_POST['dada']) ? $_POST['dada'] : ""; // This is on line 13
// Before using $_POST['value'] if (isset($_POST['value'])) { // Instructions if $_POST['value'] exist }