How can I get rid of this PHP notice?

Question

I am trying to improve my php code sitewide which is going to take forever because I want to be able to have error reporting on to show all and have no notices.

In this little bit of code below on line 6,

$selected_ date_ month This code is just a portion of code from a larger function, so sometimes $selected_ date_ month is passed in and then it is properly set but sometimes it is not. When it is not how would I prevent a NOTICE for $selected_ date_ month not being set?

//month dropdown box
$arr_month = array('January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December');
$date_combo .= ' <select name="' . $pre . 'month" class="' .$style. '"><option value="">Month</option>';
$i = 0;
for ($i = 0; $i <= 11; $i++) {
    $date_combo .= " <option ";
    if ($i + 1 == $selected_date_month) {
        $date_combo .= " selected ";
    }
    $date_combo .= " value='" . str_pad($i + 1, 2, "0", STR_PAD_LEFT) . "'>$arr_month[$i]</option>";
}
$date_combo .= "</select>";

Answer 1

if (isset($selected_date_month) && $i + 1 == $selected_date_month) {
                $date_combo .= " selected ";
        }

In this way first you check if the $selected_date_month is set and then you compare it with $i+1