title: Correctly determine if date string is a valid date in that format
body: <p>I'm receiving a date string from an API, and it is formatted as <code>yyyy-mm-dd</code>.</p>
  
  <p>I am currently using a regex to validate the string format, which works ok, but I can see some cases where it could be a correct format according to the string but actually an invalid date. i.e. <code>2013-13-01</code>, for example.</p>
  
  <p>Is there a better way in PHP to take a string such as <code>2013-13-01</code> and tell if it is a valid date or not for the format <code>yyyy-mm-dd</code>?</p>