title: PHP function - compare with the following examples
body: <ol>
  <li>compare with the two similar examples and please tell me why?</li>
  <li><p>please explain it clearly.</p>
  
  <pre><code>function inc(&amp;$value, $amount = 1) {
      $value = $value + $amount;
      echo $value;    //21
  }
  
  $val = 20;
  inc($val); 
  echo "&lt;br&gt;$val";   //21
  
  echo "&lt;hr&gt;";
  
  function incSec(&amp;$value, $amount = 1) {
      $value = $value + $amount;
      echo $value;    //21
  }
  
  incSec($valSec = 20);   
  echo "&lt;br&gt;$valSec";    //20
  </code></pre></li>
  </ol>
  
  <p>I'm trying to test, but can't get the reason.</p>