title: sum an especial index in array
body: <p>How can I sum an especial index in array for example I wanna sum -age(it's an index) in this array:</p>
  
  <pre><code>$users = array(
  
      "user1" =&gt; array("name" =&gt; "Loghman Avand", "age" =&gt; 26),
      "user2" =&gt; array("name" =&gt; "Sara Alavi", "age" =&gt; 34),
      "user3"=&gt; array("name" =&gt; "Hossein Ahmadi", "age" =&gt; 3)
  );
  </code></pre>