$big = "This is a sentence that has more than 100 characters in it, and I want to return a string of only full words that is no more than 100 characters!"

$small = some_function($big);

$small = "This is a sentence that has more than 100 characters in it, and I want to return a string of only"

$pos=strpos($content, ' ', 200);
substr($content,0,$pos );

//truncate a string only at a whitespace (by nogdog)
function truncate($text, $length) {
   $length = abs((int)$length);
   if(strlen($text) > $length) {
      $text = preg_replace("/^(.{1,$length})(s.*|$)/s", '\1...', $text);
   }
   return($text);
}

function limitString($string, $limit = 100) {
    // Return early if the string is already shorter than the limit
    if(strlen($string) < $limit) {return $string;}

    $regex = "/(.{1,$limit})b/";
    preg_match($regex, $string, $matches);
    return $matches[1];
}

function truncate($str, $len) {
  $tail = max(0, $len-10);
  $trunk = substr($str, 0, $tail);
  $trunk .= strrev(preg_replace('~^..+?[s,:]b|^...~', '...', strrev(substr($str, $tail, $len-$tail))));
  return $trunk;
}

<?PHP
$big = "This is a sentence that has more than 100 characters in it, and I want to return a string of only full words that is no more than 100 characters!";
$small = some_function($big);
echo $small;

function some_function($string){
     $string = substr($string,0,100);
     $string = substr($string,0,strrpos($string," "));
     return $string;
}
?>

list($short) = explode("n",wordwrap($string,100));

function text_cut($text, $length = 200, $dots = true) {
    $text = trim(preg_replace('#[snrt]{2,}#', ' ', $text));
    $text_temp = $text;
    while (substr($text, $length, 1) != " ") { $length++; if ($length > strlen($text)) { break; } }
    $text = substr($text, 0, $length);
    return $text . ( ( $dots == true && $text != '' && strlen($text_temp) > $length ) ? '...' : '');
}

/**
 * get_words_until() Returns a string of delimited text parts up to a certain length
 * If the "words" are too long to limit, it just slices em up to the limit with an ellipsis "..."
 *
 * @param $paragraph - The text you want to Parse
 * @param $limit - The maximum character length, e.g. 160 chars for SMS
 * @param string $delimiter - Use ' ' for words and '. ' for sentences (abbreviation bug) :)
 * @param null $ellipsis - Use '...' or ' (more)' - Still respects character limit
 *
 * @return string
 */
function get_words_until($paragraph, $limit, $delimiter = ' ', $ellipsis = null)
{
    $parts = explode($delimiter, $paragraph);

    $preview = "";

    if ($ellipsis) {
        $limit = $limit - strlen($ellipsis);
    }

    foreach ($parts as $part) {
        $to_add = $part . $delimiter;
        if (strlen($preview . trim($to_add)) <= $limit) { // Can the part fit?
            $preview .= $to_add;
            continue;
        }
        if (!strlen($preview)) { // Is preview blank?
            $preview = substr($part, 0, $limit - 3) . '...'; // Forced ellipsis
            break;
        }
    }

    return trim($preview) . $ellipsis;
}

$big = "This is a sentence that has more than 100 characters in it, and I want to return a string of only full words that is no more than 100 characters!"

$small = get_words_until($big, 100);

php >  $ali = "ali veli krbin yz doksan esikesiksld sjkas laksjald lksjd asldkjadlkajsdlakjlksjdlkaj aslkdj alkdjs akdljsalkdj ";
php > list($short) = explode("n",wordwrap($ali ,50));
php > var_dump($short);
string(42) "ali veli krbin yz doksan esikesiksld sjkas"
php > $ali ='';
php > list($short) = explode("n",wordwrap($ali ,50));
php > var_dump($short);
string(0) ""

//trim message to 100 characters, regardless of where it cuts off
$msgTrimmed = mb_substr($var,0,100);

//find the index of the last space in the trimmed message
$lastSpace = strrpos($msgTrimmed, ' ', 0);

//now trim the message at the last space so we don't cut it off in the middle of a word
echo mb_substr($msgTrimmed,0,$lastSpace)

function str_limit($str, $len = 100, $end = '...')
{
    if(strlen($str) < $len)
    {
        return $str;
    }

    $str = preg_replace("/s+/", ' ', str_replace(array("rn", "r", "n"), ' ', $str));

    if(strlen($str) <= $len)
    {
        return $str;
    }

    $out = '';
    foreach(explode(' ', trim($str)) as $val)
    {
        $out .= $val . ' ';

        if(strlen($out) >= $len)
        {
            $out = trim($out);
            return (strlen($out) == strlen($str)) ? $out : $out . $end;
        }
    }
}