Fejl i dato funktion?

Denne funktion skriver winter men bør den ikke skrive spring nu?

function getSeason($hemisphere, $month=null)
{
    $month = is_null($month) ? date('m') : $month;

    /*** northern hemisphere seasons ***/
    $northern=array(
    'summer' => array(6, 7, 8),
    'autumn' => array(9, 10, 11),
    'winter' => array(12, 1, 2),
    'spring' => array(3, 4, 5)
    );

    /*** loop over the hemisphere ***/
    foreach($$hemisphere as $key=>$val)
    {
        if(in_array($month, $val))
        {
            return $key;
        }
    }
    return false;
}

/*** show season of december in the northern hemisphere  ***/
echo getSeason('northern', 12);