WordPress自动获取文章第一张图片作为缩略图支持外链图片

1、在 functions.php 文件增加：

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);
    $first_img = $matches[1][0];

    if(empty($first_img)) {
    $first_img = "/path/to/default.png";
    }
    return $first_img;
}

2、模板文件调用：

if ( get_the_post_thumbnail($post_id) != '' ) {

    echo '<a href="'; the_permalink(); echo '" class="thumbnail-wrapper">';
    the_post_thumbnail();
    echo '</a>';

    } else {

    echo '<a href="'; the_permalink(); echo '" class="thumbnail-wrapper">';
    echo '<img src="';
    echo catch_that_image();
    echo '" alt="" />';
    echo '</a>';

}