title: YII_TRACE_LEVEL in index.php YII?
body: <p>In my index.php file , </p>
  
  <p>I know that this  YII_DEBUG is use to show any error on browser screen</p>
  
  <pre><code>defined('YII_DEBUG') or define('YII_DEBUG',true);
  </code></pre>
  
  <p>What is mean this below line YII_TRACE_LEVEL ?</p>
  
  <pre><code>defined('YII_TRACE_LEVEL') or define('YII_TRACE_LEVEL',3);
  </code></pre>
  
  <p>1)What is YII_TRACE_LEVEL?</p>
  
  <p>2)What is 3 meaning in this? </p>