WordPress随机获取文章里的一张图片

function catch_that_image() {
    global $post, $posts;
    $first_img = '';
    ob_start();
    ob_end_clean();
    $output = preg_match_all('/<img.+?src=[\'"]([^\'"]+)[\'"].*?>/i', $post->post_content, $matches);
    shuffle($matches[1]);
    $first_img = $matches[1][0];
    if(empty($first_img)) {
        $first_img = '.../images_pub/pub_' . rand(1, 300) . '.jpg';
    }
    return $first_img;
}