Search a bit, the most recommended is:
$year = "2009"; // date("Y");
$week = "45"; // date("W");
$firstDayOfWeek = strtotime($year."W".str_pad($week,2,"0",STR_PAD_LEFT));
print("The first day of week ".$week." of ".$year." is ".date("D, d-m-Y",$firstDayOfWeek));
It basically comes down to having strtotime do the work for you. You fill out "2009W45" and it will receive a date for you. Pitfall here is that the week should be in a two-digit format. So week 1 should be 01, so str_pad will fill in zero.
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