echo get_hashtags($string);

function get_hashtags($string, $str = 1) {
    preg_match_all('/#(\w+)/',$string,$matches);
    $i = 0;
    if ($str) {
        foreach ($matches[1] as $match) {
            $count = count($matches[1]);
            $keywords .= "$match";
            $i++;
            if ($count > $i) $keywords .= ", ";
        }
    } else {
        foreach ($matches[1] as $match) {
            $keyword[] = $match;
        }
        $keywords = $keyword;
    }
    return $keywords;
}