Gets the plain-text version of $this->content.
string $content: A (HTML) string.
string The $content with all HTML tags stripped and all HTML entities replaced with their actual characters.
protected function getPlainTextContent($content) {
// Strip all HTML tags.
$content = strip_tags($content);
// Decode all HTML entities (e.g., ' ') into characters.
$content = html_entity_decode($content, ENT_QUOTES, 'UTF-8');
return $content;
}