wordpress 获取文章内的第一张图片

<?php
// 获取文章第一张图片
function catch_that_image() {
    global $post, $posts;
    $first_img = '';
    ob_start();
    ob_end_clean();
    $output = preg_match_all('/<img.+?src=[\'"]([^\'"]+)[\'"].*?>/i', $post->post_content, $matches);
    if ($output) {
        $first_img = $matches[1][0];
    }

    if (empty($first_img)) {
        $first_img = bloginfo('template_url') . "/assets/images/default_image.jpeg";
    }
    return $first_img;
}