code: protected function generateOption($val, $index)
      {
          $checked = '';
          if (is_array($this->varValue) && in_array($val['value'], $this->varValue)) {
              $checked = ' checked="checked"';
          }
  
          return sprintf(
              '<span class="%1$s opt_%2$s">' .
              '<input type="checkbox" name="%8$s[]" id="opt_%3$s" class="checkbox" value="%4$s"%5$s%6$s ' .
              '<label id="lbl_%3$s" for="opt_%3$s">%7$s</label></span>',
              // @codingStandardsIgnoreStart - Keep the comments.
              $this->getClassForOption($index),             // 1
              $index,                                       // 2
              $this->strName.'_'.$index,                    // 3
              $val['value'],                                // 4
              $checked,                                     // 5
              $this->getAttributes() . $this->strTagEnding, // 6
              $val['label'],                                // 7
              $this->strName                                // 8
              // @codingStandardsIgnoreEnd
          );
      }
docs: Generate a single checkbox.
  
  @param array $val   The value array (needs keys "value" and "label").
  
  @param int   $index The sequence number of this option (used for even/odd determination).
  
  @return string