title: How to sum array values by defined keys?
body: <pre><code>$array[0] = 100;
  $array[1] = 10;
  $array[2] = 15;
  </code></pre>
  
  <p>How can I get sum of array by its keys like <code>key = (0,1)</code> so the <code>sum = 110</code> ?</p>