title: Get first day of week in "Y-m-d" of given week number in the year
body: <p>im trying to get the first day of a given week based on inputting a week number and year. im currently using this for the current week and year and works like a charm.</p>
  
  <pre><code>$week = date('W');
  $year = date('Y');
  $date1 = date('Y-m-d', strtotime($year."W".$week.'1'));
  </code></pre>
  
  <p>however when i try to input my own week and year it doesn't work</p>
  
  <pre><code>$week = 7;
  $year = 2013;
  $date1 = date('Y-m-d', strtotime($year."W".$week.'1'));
  </code></pre>
  
  <p>what type if string should i be using for strtotime? how come providing my own date doesn't work?</p>