Here is a little function taking a week number and a year as parameters, and returning the date of this week's monday. If no parameters are given, it will use the current week and year.

function getMonday($week = null, $year = null)
{
    $week || $week = date('W');
    $year || $year = date('Y');

    return date('Y-m-d', strtotime($year.'W'.str_pad($week, 2, '0', STR_PAD_LEFT)));
}