Contents of PHP function rendered as text in html document

Problem: Content of user function is rendered as plain text.

This is my class.aktionen.php:

<?

class user_aktionen {

  private $conf;

  function main($content,$conf) {
    $this->conf = $conf;

    return 'test';
  }

}
?>

This is my TS:

includeLibs.user_aktionen = fileadmin/templates/class.aktionen.php

lib.offers = USER_INT
lib.offers {
  userFunc = user_aktionen->main
}

I have mapped lib.offers with TemplaVoila onto a div element. The rendered output of my HTML-Document is

<?

class user_aktionen {

  private $conf;

  function main($content,$conf) {
    $this->conf = $conf;

    return 'test';
  }

}
?><?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html
     PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
     "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xml:lang="de-DE" lang="de-DE" xmlns="http://www.w3.org/1999/xhtml">

I'm using Templavoila 1.8.0 and Typo3 4.7.10.

What I'm doing wrong?

Is it a permission problem? Should I use Templavoila 1.7 instead? I tried

$TYPO3_CONF_VARS['FE']['debug'] = '1';
$TYPO3_CONF_VARS['SYS']['devIPmask'] = '127.0.0.1'; // your real IP here
$TYPO3_CONF_VARS['SYS']['displayErrors'] = '2';

and php_flag display_errors on in .htaccess but I don't see an error at all.

Solution

Thanks to Charles here is the solution:

Short tags (<? ?>) are not configured on the server. Switching to <?php ?> solved the problem.