title: usort parameter issue
body: <p>I have the following code, which tries to sort an array of Products by it's creation date:</p>
  
  <pre><code>private function sortProductsByDate(Product $a, Product $b)
  {
     if ($a-&gt;getCreated() == $b-&gt;getCreated()) {
        return 0;
     }
     return ($a-&gt;getCreated() &lt; $b-&gt;getCreated()) ? -1 : 1;
  }
  
  /**
   * Get the most 4 recent items
   *
   * @return \Doctrine\Common\Collections\Collection 
   */
  public function getMostRecentItems()
  {
     $userMostRecentItems = array();
     $products = $this-&gt;getProducts();
     usort($products, "sortProductsByDate");
  
     foreach ($this-&gt;getProducts() as $product) {
        ladybug_dump($product-&gt;getCreated());
     }
  
  
     $mostRecentItems = $this-&gt;products;
     return $this-&gt;isLocked;
  }
  </code></pre>
  
  <p>Why does this gives me this error:</p>
  
  <pre><code>Warning: usort() expects parameter 1 to be array, object given 
  </code></pre>
  
  <p>Ideas?</p>