Since I'll always need this function again, I decided to write it down. There is a nice approach on Stack Overflow, that unfortunately has it's problems.

function truncate($text, $chars = 25) {
    $text = $text." ";
    $text = substr($text,0,$chars);
    $text = substr($text,0,strrpos($text,' '));
    $text = $text."...";
    return $text;
}

1. It always runs every input through the functions, no matter if necessary or not.
2. It always sets dots behind a sentence, no matter how short it is.

My simple addition with an if-statement:

function truncate($text, $chars = 25) {
  if (strlen($text) > $chars) {
    // leave me alone if I'm short enough
    $text = $text.' ';
    $text = substr($text, 0, $chars);
    $text = substr($text, 0, strrpos($text, ' '));
    $text = $text.'...';
  }
  return $text;
}