How to fix the missing captions from the gallery without re-selecting each image

Here is a simple snippet that makes the captions pop right back into the galleries, without having to change the existing content.

declare( strict_types = 1 );
namespace IuliaCazanSnippets\GalleryFix;
\add_filter( 'render_block', __NAMESPACE__ . '\\block_wrapper_image', 30, 2 );

/**
 * Fix the old gallery captions.
 * Snippet by Iulia Cazan - https://iuliacazan.ro.
 *
 * @param  string $block_content Block content.
 * @param  array  $block         Block attributes.
 * @return string
 */
function block_wrapper_image( string $block_content, array $block ): string {
	if ( 'core/image' !== $block['blockName'] ) {
		// Fail-fast, this is not the block we are targetting.
		return $block_content;
	}

	if ( empty( $block['attrs']['id'] ) ) {
		// Fail-fast, the image ID is missing, we cannot do anything about it.
		return $block_content;
	}

	if ( ! substr_count( $block_content, '<figcaption' ) ) {
		// The block output lacks the expected markup.
		$caption = \wp_get_attachment_caption( $block['attrs']['id'] );
		if ( ! empty( $caption ) ) {
			// We could get the image caption, let's use it in the output.
			$block_content = str_replace( '</figure>', '<figcaption>' . $caption . '</figcaption></figure>', $block_content );
		}
	}

	return $block_content;
}

Most of the time, the easiest way to add a snippet on your website is inside your child theme’s functions.php file.

I hope this snippet comes in handy. Check out other tips & tricks and stay tuned for more in my future posts.
Happy coding!