function primeira_imagem() {
    global $post, $posts;
    $first_img = '';
    ob_start();
    ob_end_clean();
    $output    = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);

    if($output >= 1) {
        $first_img = $matches[1][0];
    }

    return $first_img;
}