title: PHP - Why is DateTime object copied by reference in my code?
body: <p>Why in this code my DateTime object was copied by reference it seems?<br>
  Here's my code:</p>
  
  <pre><code>&lt;?php
  
  date_default_timezone_set('UTC');
  
  $dt1 = new \DateTime('2015-03-15');
  $dt2 = $dt1;
  $dt2 = $dt2-&gt;modify('-1 year');
  
  echo $dt1-&gt;format('c') . PHP_EOL;
  echo $dt2-&gt;format('c');
  
  ?&gt;
  </code></pre>
  
  <p>I was expecting:</p>
  
  <pre><code>2015-03-15T00:00:00+00:00
  2014-03-15T00:00:00+00:00
  </code></pre>
  
  <p>But I got this:  </p>
  
  <pre><code>2014-03-15T00:00:00+00:00
  2014-03-15T00:00:00+00:00
  </code></pre>