title: Calculate if 2 or more dates overlap
body: <p>Is there an efficient way of calculating whether 2 or more dates overlap in PHP?</p>
  
  <p>Ex. </p>
  
  <ul>
  <li><strong>Date1:</strong> Start1 = 2012-03-10 / End1 = 2012-05-10</li>
  <li><strong>Date2:</strong> Start2 = 2012-04-25 / End2 = 2012-06-01</li>
  <li><strong>Date3:</strong> Start3 = 2012-07-15 / End3 = 2012-08-20</li>
  </ul>
  
  <p>In the example above Date1 and Date2 overlaps, Date3 is OK.</p>