title: Magento. OnBlockToHtml Before-After
body: <p>In magento there are 2 events:</p>
  
  <p>core_block_abstract_to_html_before
  core_block_abstract_to_html_after</p>
  
  <p>They are called every time when toHtml method is called. What I want is to echo to output some valuable data, BUT I get following error:</p>
  
  <pre><code>HEADERS ALREADY SENT
  </code></pre>
  
  <p>So is there any way to append to output?</p>