title: I have a weird bug in controller with Symfony4
body: <p>This is my controller</p>
  
  <pre><code>    $entretien = new Entretien();
      $form3 = $this-&gt;createForm(EntretienType::class, $entretien); // entretien type has a attribute debutRdv
      $form3-&gt;add('duree', TimeType::class, [
          'placeholder' =&gt; [
              'hour' =&gt; 'Heure', 'minute' =&gt; 'Minute', 'second' =&gt; 'Seconde',
          ],
          'with_seconds' =&gt; true,
          "mapped" =&gt; false,
      ]);
  
  $form3-&gt;handleRequest($request);
  
      if($form3-&gt;isSubmitted())
                  {
                      $debutRdv= $entretien-&gt;getDebutRdv(); // getdata from form
  
                      $duree =$form3-&gt;get("duree")-&gt;getData();
  
                      $duree = $duree-&gt;format('\P\TH\Hi\Ms\S');
  /*------------Here the probleme--------*/
                          $finRdv = $debutRdv-&gt;add(new \DateInterval($duree));
  /*-------------------------------------*/
  ...
  </code></pre>
  
  <p>The problem is when I write $debutRdv->add(...) the attribute $debutRdv changes to $finRdv like I've done $debutRdv = $finRdv;</p>
  
  <p>For example
  <strong>$debutRdv: "2019-03-25 16:30:00"
  $duree : "0:30:00"</strong></p>
  
  <p>When I write this</p>
  
  <pre><code>$finRdv = $debutRdv-&gt;add(new \DateInterval($duree));
  </code></pre>
  
  <p><strong>$duree</strong> and <strong>$debutRdv</strong> change to <strong>2019-03-25 17:00:00</strong>
  but I only want <strong>$duree</strong> got <strong>2019-03-25 17:00:00</strong></p>
  
  <p>I want to solve this problem because 
  $entretien->getDebutRdv() also changes to  <strong>2019-03-25 17:00:00</strong> Like I have done $entretien->setDebutRdv($duree)</p>