Making custom field appear in captions for image attachments (Wordpress) - php

Question

I'm a designer (not a developer) working on a Wordpress site for a customer. Using the ACF-plugin I've set up a custom field on media files for photo credits. This works fine on featured images, where I can call it in single.php like this:
$post_thumbnail = get_post(get_post_thumbnail_id());
$credit = get_field('media_credit', $post_thumbnail);
if($credit):
echo '<div class="media-credit"><p>Photo: '.$credit.'</p></div>';
endif;
So I know the custom field works, and outputs the right data. However, I can't get it to work on image attachments in posts. What I have is this:
add_filter( 'img_caption_shortcode', 'my_img_caption_shortcode', 10, 3 );
function my_img_caption_shortcode( $empty, $attr, $content ){
$attr = shortcode_atts( array(
'id' => '',
'align' => 'alignnone',
'width' => '',
'caption' => ''
), $attr );
if ( 1 > (int) $attr['width'] || empty( $attr['caption'] ) ) {
return '';
}
if ( $attr['id'] ) {
$attr['id'] = 'id="' . esc_attr( $attr['id'] ) . '" ';
}
//OUTPUT CREDIT
$photographer = get_field( 'media_credit', $attachment_id );
if ($photographer):$media_byline = '<br/><span class="media-credit">Photo: '.$photographer.'</span>';endif;
return '<div ' . $attr['id']
. 'class="wp-caption ' . esc_attr( $attr['align'] ) . '" '
. do_shortcode( $content )
. '<p class="wp-caption-text">' . $attr['caption'] . '' . $media_byline . '</p>'
. '</div>';
}
If I remove the if-statement in OUTPUT it shows «Photo: » within the captions, after the text like it should, but it doesn't get any data. What am I missing?
(BTW – I know there are plugins that outputs image credits, but they tend to have styles and features I have to override, resulting in a spaghetti mess I'd hate to hand over to the next guy working on this site.)

Answer 1

Finally got this to work! :-D Instead of using $attachment_id, I got the ID from $attr, and then stripped the 'attachment_' prefix from output.
I also made separate fields for photographer and bureau, but I guess that's beside the point.
Here is the code:
function my_img_caption_shortcode( $empty, $attr, $content ){
$attr = shortcode_atts( array(
'id' => '',
'align' => 'alignnone',
'width' => '',
'caption' => ''
), $attr );
if ( 1 > (int) $attr['width'] || empty( $attr['caption'] ) ) {
return '';
}
$credit_id = $attr['id'];
$credit_id = str_replace( 'attachment_', '', $credit_id );
$photographer = get_field( 'media_credit', $credit_id );
$bureau_credit = get_field( 'media_bureau', $credit_id );
if ( $photographer && $bureau_credit ): $dash = ' / ';
endif;
if ( $photographer || $bureau_credit ): $media_byline = '<br/><span class="media-credit">PHOTO: '
. $photographer . ''
. $dash . '<span class="bureau-credit">'
. $bureau_credit
. '</span></span>';
endif;
return '<div id="attachment_' . $credit_id . '"'
. 'class="wp-caption ' . esc_attr( $attr['align'] ) . '" '
. do_shortcode( $content )
. '<p class="wp-caption-text">' . $attr['caption'] . '' . $media_byline . '</p>'
. '</div>';
}
add_filter( 'img_caption_shortcode', 'my_img_caption_shortcode', 10, 3 );
This solution is something I lifted from the AFC Media Credit-plugin, so credits to the developer.
I hope this is useful for anybody who wants to achieve something similar.