limiting string length by number of charachters while preserving whole words

Limit_string.php

<?php
/**
 * replace array Values (string) with length values
 * @param array $string
 */
function get_string_length(&$string){

    if ((is_string($string)) || (is_integer($string)))
        $string = strlen($string);
}


/**
 * Trimg string based on number on characters without cutting off words
 * @param string $string
 * @param int $length
 * @param string|null $trailing
 * @return string
 */
function limit_string_length($string,$length=-1,$trailing = null){
    if ((strlen($string) > $length) && ($length > 0)){

        /* explode string in an array*/
        $particles = explode(' ',$string);

        /*duplicate array*/
        $particles_values = $particles;

        /*replace all strings with their length*/
        array_walk($particles_values,'get_string_length');

        /*while the sum of all characters bigger than the desired length  */
        while (array_sum($particles_values) > $length){
            /*pop one word from the end of the array*/
            array_pop($particles_values);
        }

        /*splice our original array to the length of our dummy array*/
        $new_particles = array_splice($particles,0,count($particles_values));

        /*glue back the array and append to it the desired trailing*/
        return implode(' ',$new_particles).$trailing;
    }
    return $string;
}