This function will truncate strings only at word breaks which can be used to show a teaser for complete article without breaking words.

truncate_text.php

<?php

// Original PHP code by Chirp Internet: www.chirp.com.au   
// Please acknowledge use of this code by including this header.   
function myTruncate($string, $limit, $break=".", $pad="...") {   
    // return with no change if string is shorter than $limit    
    if(strlen($string) <= $limit)   
        return $string;   
      
    // is $break present between $limit and the end of the string?    
    if(false !== ($breakpoint = strpos($string, $break, $limit))) {  
        if($breakpoint < strlen($string) - 1) {   
            $string = substr($string, 0, $breakpoint) . $pad;   
        }   
    }  
    return $string;   
}  
/***** Example ****/  
$short_string=myTruncate($long_string, 100, ' ');