中文，俄文字符的PHP字数限制功能（UTF8）

I have a useful function that grabs the text from the long description of products, news, etc and shows limited number of words to create news or product snapshots . Now the problem is when the long description is in Chinese, Russian, Arabic (none ASCII) this function breaks.

How can I enforce UT8 encoding into this function or is there any alternative to show limited number of words from a long description?

function wordLimit($string,$limitNum){
 if (strlen($string) > $limitNum){
   $string = substr($string, 0, strrpos(substr($string, 0, $limitNum), ' ')) . '...';
 }
   return $string;
}