Only show read more when necessary

Hi,

I have this code, and I want to only show the “read more” when the actual content is more than the limit I have set.

<?php $content = get_the_content();
      $content = strip_tags($content);
      echo substr($content, 0, 1500);
?>…<a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>">read more.</a>

And I want to put a if and if / else /endif in there like I do for my titles like this:

<?php $title  = the_title('','',false);
   if(strlen($title) > 20):
  echo trim(substr($title, 0, 25)).'...';
  else:
  echo $title;
  endif;
  ?>

But I cant get it to work 🙁
Anyone who can help me with this?