<?php
    $string = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Quisque nec elit dui, nec fermentum velit. Nullam congue ipsum ac quam auctor nec massa nunc.";

    $output = array();
    while (strlen($string) > 50) {
        $index = strpos($string, ' ', 50);
        $output[] = trim(substr($string, 0, $index));
        $string = substr($string, $index);
    }
    $output[] = trim($string);

    var_dump($output);

array(3) {
  [0]=>
  string(50) "Lorem ipsum dolor sit amet, consectetur adipiscing"
  [1]=>
  string(55) "elit. Quisque nec elit dui, nec fermentum velit. Nullam"
  [2]=>
  string(43) "congue ipsum ac quam auctor nec massa nunc."
}