This should work now:
$year = 2007;
$week = 42;
$day = 1; /* this is the value of Mon in the week day */
/* timestamp of the first day of the year*/
$timestamp_year = mktime(0,0,0,1,1,$year);
/* the timestamp of the 42 week in 2007 */
$timestamp_nr_week = strtotime("+{$week} week",$timestamp_year);
/* determine the day of the week*/
$day_of_the_week = (int) date("w",$timestamp_nr_week);
if ($day_of_the_week>$day) {
echo date("D, d M Y",strtotime("-".($day_of_the_week-$day)." day",$timestamp_nr_week));
} elseif ($day_of_the_week<$day) {
echo date("D, d M Y",strtotime("+".($day-$day_of_the_week)." day",$timestamp_nr_week));
} else {
echo date("D, d M Y",$timestamp_nr_week);
}