Why not fire the save_post event?

Question

The save_post action hook fires after a post is saved: every time, even for autosaves.

If you look at the reference for save_post, you’ll see that the callback receives three parameters: the first is an integer $post_ID, the second is a WP_Post object $post, and the third is a boolean $update. None of the parameters are a string of the content.

Based upon your code, it appears that you are trying to add a filter to the_content. So your code should be:

function action_function_name_13( $content ) {
  return '<div>' . $content . '</div>';
}
add_filter( 'the_content', 'action_function_name_13' );

If this isn’t what you’re trying to do, look through the hook reference for a hook that describes what you’re trying and update your question.

Here’s a way to modify the post content after it has been saved to the database.

//* save_post fires *after* the post has been saved to the database
add_action( 'save_post', 'wpse_266719_save_post', 10, 3 );
function wpse_266719_save_post( $post_ID, $post, $update ) {
  //* Remove action so it doesn't fire on wp_update_post()
  remove_action( 'save_post', 'wpse_266719_save_post', 10 ); 
  $id = $post->ID;
  $content = htmlspecialchars( $post->post_content, ENT_QUOTES, 'UTF-8', false );
  $postarr = array( 'ID' => $id, 'post_content' => $content, );
  wp_update_post( $postarr );
}