title: Magento query __toString() like Zend
body: <p>i can return this like a string?
  Similar Zend Framework ->__toString()</p>
  
  <p>here i select my categories (just name) when is active...</p>
  
  <pre><code>$categories = Mage::getModel('catalog/category')
          -&gt;getCollection()
          -&gt;addAttributeToSelect('name')
          -&gt;addFieldToFilter('is_active', 1);
  </code></pre>
  
  <p>i want see that like string sql query "SELECT * ..."</p>