title: add key to associative array if key is not found
body: <p>Given a PHP associative array like this one:</p>
  
  <pre><code>$a = array(
      'color' =&gt; 'red',
      'taste' =&gt; 'sweet',
      'shape' =&gt; 'round',
      'name'  =&gt; 'apple'
  );
  </code></pre>
  
  <p>I want to search a key and, if not found, I want to add 'myKey'=>0. Which is the best way to do such a thing?</p>