I have needed to do things like that in the past, so what I have done, and I am sure that this is not the best way, but I have substringed the week number, and wrote a conditional statement
if ($week < 10){
$week = "0 . $week";
}
This places a 0 in front of the number.
then you can append it to the 2003
ie. $year = $year . $week
this gives you 200302
there is probably a much better way, but this seems to work for me everytime