title: php array_filter without key preservation
body: <p>if i filter an array with array_filter to eliminate null values, keys are preserved and this generated "holes" in the array. Eg:</p>
  
  <pre><code>The filtered version of
      [0] =&gt; 'foo'
      [1] =&gt;  null
      [2] =&gt; 'bar'
  is 
      [0] =&gt; 'foo'
      [2] =&gt; 'bar'
  </code></pre>
  
  <p>How can i get, instead</p>
  
  <pre><code>[0] =&gt; 'foo'
  [1] =&gt; 'bar'
  </code></pre>
  
  <p>?</p>