First X words in formatted string

function first_words($string, $num)
{
        $words = str_word_count($string, 2);
        $firstwords = array_slice( $words, 0, $num);
        return  implode(' ', $firstwords);
}

I use this PHP function to get the first X number of words from a field in a mysql table.
In an ideal world it works, however if the field contains html tags "<p>" etc... they are included in the string that is returned.

I want to ignore all formatting and simply get the first 10 words in the field