Try making sure your function showContent is ‘returning’ the output instead of just echoing it out. That might help as your text will then be in the correct location of the page output.
ie:
function showContent($content = '') {
$output .= '<p>Hello World</p>';
return $output;
}
Good luck!
A gui would be great. Also, is it possible to include a range? Say include=”160-250″, for example?