str_word_count() function doesn't display Arabic language properly

I've made the next function to return a specific number of words from a text:

function brief_text($text, $num_words = 50) {
    $words = str_word_count($text, 1);
    $required_words = array_slice($words, 0, $num_words);
    return implode(" ", $required_words);
}

and it works pretty well with English language but when I try to use it with Arabic language it fails and doesn't return words as expected. For example:

$text_en = "Cairo is the capital of Egypt and Paris is the capital of France";
echo brief_text($text_en, 10);

will output Cairo is the capital of Egypt and Paris is the while

$text_ar = "القاهرة هى عاصمة مصر وباريس هى عاصمة فرنسا";
echo brief_text($text_ar, 10); 

will output � � � � � � � � � �.

I know that the problem is with the str_word_count function but I don't know how to fix it.

UPDATE

I have already written another function that works pretty good with both English and Arabic languages, but I was looking for a solution for the problem caused by str_word_count() function when using with Arabic. Anyway here is my other function:

    function brief_text($string, $number_of_required_words = 50) {
        $string = trim(preg_replace('/\s+/', ' ', $string));
        $words = explode(" ", $string);
        $required_words = array_slice($words, 0, $number_of_required_words); // get sepecific number of elements from the array
        return implode(" ", $required_words);
    }