src/Debug/Output/Html.php
protected function visualWhiteSpace($str)
    {
        // display \r, \n, & \t
        $str = \preg_replace_callback('/(\r\n|\r|\n)/', array($this, 'visualWhiteSpaceCallback'), $str);
        $str = \preg_replace('#(<br />)?\n$#', '', $str);
        $str = \str_replace("\t", '<span class="ws_t">'."\t".'</span>', $str);
        return $str;
    }
protected function visualWhiteSpace($str)
    {
        // display \r, \n, & \t
        $str = \preg_replace_callback('/(\r\n|\r|\n)/', array($this, 'visualWhiteSpaceCallback'), $str);
        $str = \preg_replace('#(<br />)?\n$#', '', $str);
        $str = \str_replace("\t", '<span class="ws_t">'."\t".'</span>', $str);
        return $str;
    }
Add whitespace markup

@param string $str string which to add whitespace html markup

@return string