php - Getting first image from post

Solution:

I understand that you want to check if there is a custom image for that post (from the ACF field), and if there isn't take the first image from the post content.

I also understand (didn't check) that your current code works for the second case (get the first image from the post), so all you need is to add a query for the ACF image.

I would try something like this:

function catch_that_image() {
    global $post;
    
    $acf_image = get_field('your_acf_image_field_name');
    if (!empty($acf_image)) {
            return $image['url'];
    } else {
        $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;
    }
}

Please note that in WordPress there are many advantages for using the native wp_get_attachment_image() with an image ID from ACF Image fields - you can find more information here: https://www.advancedcustomfields.com/resources/image/

Solution:

@Hillel, thanks.

I tried this instead. But my return result all in array. Is there something wrong. I modified from other code.

/**
     * Filters the title and alt message for thumbnails.
     *
     * @since   2.3.0
     *
     * @param   string  $post_title     Post tile used as thumbnail alt and title
     * @param   object  $result         Post Object
     */
    $post_title = apply_filters( 'tptn_thumb_title', $post_title, $result );

    $output    = '';
    $postimage = '';
    $pick      = '';
    if(! $postimage)
    {
        $args = [
                    'numberposts' => 1,
                    'order' => 'ASC',
                    'post_parent' => $post_title,
                    'post_type' => 'attachment',
                    'post_mime_type' => 'image'
                ];
                $post_attachments = get_children($args);
    
                if ( ! empty($post_attachments) ) {
                    $output = array_shift($post_attachments->url);
    
                    return [
                        $postimage => get_attached_file($output->url),
                        $pick => get_post_meta($postimage->ID, '_wp_attachment_image_alt', true)
                    ];
                }
    }

Solution:

I try out this instead.

 if(!$postimage)
    {
        $header_image  = get_field( 'article_header_image', $result );
        if($header_image ){
            $postimage = ( $postimage['url'] );
            $pick .= 'first';
        }
        else{
            $postimage = tptn_get_first_image( $result->ID, $args['thumb_width'], $args['thumb_height'] );  // Get the first image.
            $pick      = 'firstchild';
        }
    }

How do I parse to URL?

https://i.stack.imgur.com/j34BI.jpg

I tried the edited code. Not sure why it took the image randomly. Some is correct some is not.