title: Php: Add variable amount days to Y-m-d date format
body: <p>i'm trying to add days to a date with the 'Y-m-d' format:</p>
  
  <pre><code>$oldDate = '2013-05-15';
  $newDate = date('Y-m-d', strtotime($oldDate. " + 5 days"));
  </code></pre>
  
  <p>This ouputs '2013-5-20', but below:</p>
  
  <pre><code>$oldDate = '2013-05-15';
  $addedDays = 5;
  $newDate = date('Y-m-d', strtotime($oldDate. " + $addedDays days"));
  </code></pre>
  
  <p>doesn't work, it only outputs '1970-01-01', which doesn't make sense because i only tried to put the days to be added in a variable. They're basically the same code. I appreciate the help trying to understand this. Thanks!  </p>