title: PHP is confused when adding and concatenating
body: <p>I have the following code:</p>
  
  <pre><code>&lt;?php
  
      $a = 1;
      $b = 2;
  
      echo "sum: " .  $a + $b;
      echo "sum: " . ($a + $b);
  
  ?&gt;
  </code></pre>
  
  <p>When I execute my code I get:</p>
  
  <pre><code>2
  sum: 3
  </code></pre>
  
  <p>Why does it fail to print the string <code>"sum:"</code> in the first echo? It seems to be fine when the addition is enclosed in parentheses.</p>
  
  <p>Is this weird behaviour anywhere documented?</p>