from http://www.dannyherran.com/2010/10/trim-long-texts-by-words-in-php/

trim_text.php

<?php

function trim_text($input, $length, $ellipses = true, $strip_html = true, $output_entities = true){
  // Strip tags, if desired
  if($strip_html) {
    $input = strip_tags($input);
  }
 
  // No need to trim, already shorter than trim length
  if(strlen($input) <= $length) {
    return $input;
  }
 
  // Find last space within length
  $last_space = strrpos(substr($input, 0, $length), ' ');
  $trimmed_text = substr($input, 0, $last_space);
 
  // Add ellipses (...)
  if($ellipses){
    $trimmed_text .= '...';
  }
 
  if($output_entities){
    $trimmed_text = htmlentities($trimmed_text);
  }
 
  return $trimmed_text;
}