title: Add elements to all empty keys of an array
body: <p>I use the following code to fill all empty keys in sub-arrays with <code>``</code>:</p>
  
  <pre><code>$array = array(
          'note' =&gt; array('test', 'test1'),
          'year' =&gt; array('2011','2010', '2012'),
          'type' =&gt; array('conference', 'journal', 'conference'),
      );
  
  foreach ($array['type'] as $k =&gt; $v) {
    foreach($array as $element =&gt; $a) {
      $iterator = $array[$element];
      if(!isset($iterator[$k])){       
         $iterator[$key] = '';
      }
    }
  }
  </code></pre>
  
  <p>print_r($array);</p>
  
  <p>The problem is that it is not actually changing the elements in <code>$array</code> but in temporary variable <code>$iterator</code>.</p>
  
  <p>I know that this is a simple question but I would like to find out the best and fastest solution.</p>