<?php

// your code goes here
function check_plain($text) {
	return htmlspecialchars($text, ENT_QUOTES, 'UTF-8');
}

$str = '<strong>&amp;foo<em class="bar">bar</em></strong>';
$maxlength = 5;

$cutlength = $maxlength; while (0 < $delta = $maxlength - strlen(html_entity_decode(strip_tags(substr($str, 0, $cutlength))))) {$cutlength += $delta;}
print $cutlength;