<?php

  $date = strtotime('1 january 2006');
  echo date('r',$date)."\n";
  echo "Week Number: ".week_number($date);

  function week_number($stamp){
    return date('W', $stamp+(60*60*24));
  }

Sun, 01 Jan 2006 00:00:00 +0000
Week Number: 01