Populate html table with data rows. Chooses with select boxes

PHP
6

So put your $_POST['predmet1'] into the session when it is set.
And change the condition to check session as well:

// check POST and SESSION
if (isset($_POST['predmet1']) || isset($_SESSION['predmet1'])) {
    // get ID from POST or SESSION
    $predmet1 = isset($_POST['predmet1']) ? $_POST['predmet1'] : $_SESSION['predmet1'];
    // and store it into the session
    $_SESSION['predmet1'] = $predmet1; 
    // get rows
    $rows = get_info($db, $predmet1);
    // ... the rest of the code ...