<?php
mb_internal_encoding("UTF-8");
$text = "Принадлежность того или иного символа к буквенным определяется с учетом текущей локали.";

function tooLong($string)
{
	$text_array = preg_split("/[\s,.?!]+/", $string, 0, PREG_SPLIT_NO_EMPTY);
	print_r($text_array);
	foreach ($text_array as $k => &$str)
	{
	$length = mb_strlen($str, 'UTF-8');
	if ($length > 10)
	{
	
	$length -= 2;
	$firstLetter = mb_substr($str, 0, 1);
	$lastLetter = mb_substr($str, -1);
	$str = $firstLetter.$length.$lastLetter;
	}
	}
	$words = '';
	foreach ($text_array as $key => $value)
	{
		$words.=$value." ";
	}
	return $words;
}
print_r(tooLong($text));