title: Reindex array by values
body: <p>I have an array of the form</p>
  
  <pre><code>$a = array(
     'a' =&gt; 0,
     'b' =&gt; 0,
     'c' =&gt; 1
  );
  </code></pre>
  
  <p>and want an array of the form</p>
  
  <pre><code>array(
     0 =&gt; array('a','b'),
     1 =&gt; array('c')
  );
  </code></pre>
  
  <p>What is the most efficient way to do this?</p>