title: strtotime() converts a non existing date to another date
body: <p>I am building a timestamp from the date, month and year values entered by users.</p>
  
  <p>Suppose that the user inputs some wrong values and the date is "31-02-2012" which does not exist, then I have to get a false return. But here its converting it to another date nearby. Precisely to: "02-03-2012"..</p>
  
  <p>I dont want this to happen..</p>
  
  <pre><code>$str = "31-02-2012";
  echo date("d-m-Y",strtotime($str)); // Outputs 02-03-2012
  </code></pre>
  
  <p>Can anyone help? I dont want a timestamp to be returned if the date is not original.</p>