look at last example at http://www.php.net/ZEND_CHANGES.txt

$backtrace = debug_backtrace();

        foreach ($backtrace as $step) {
            $class = isset($step['class']) ? $step['class'] . '::' : '';

            printf(
              "%s [%s:%s]\n",
              $step['function'],
              $step['file'],
              $step['line']
            );
        }

$class is set but not used.  i think that printf call should be changed to:
            printf(
              "%s%s [%s:%s]\n",
              $class,
              $step['function'],
              $step['file'],
              $step['line']
            );

probably someone forgot to add this part, which is pretty important.