Replace missing attachments with placekitten placeholders, useful for local imports of databases without the uploads folder

local-kitten.php

<?php
/**
 * Local Kitten images.
 *
 * Filters image URLs to load files from placekitten if they aren't present locally.
 *
 * @wordpress-plugin
 * Plugin Name: Local Kitten Images
 * Version:     1.0.2
 * Description: Loads missing image files from placekitten when using local environment.
 * Author:      Human Made
 * License:     GPL v3
 */

add_filter( 'wp_get_attachment_image_src', 'localkitten_environment_attachment_image_src', 10, 2 );
add_filter( 'wp_calculate_image_srcset', 'localkitten_environment_wp_calculate_image_srcset', 10, 5 );
add_filter( 'wp_get_attachment_url', 'localkitten_wp_get_attachment_url', 10, 2 );

/**
 * Returns the placeholder image service URL, with an optional seed
 *
 * @param string $seed optional seed if the service supports it
 * @return string placeholder service URL ready for width and height appending
 */
function localkitten_get_service_url( $seed = '' ) : string {
	return 'https://placekitten.com/';

	/*if ( ! empty( $seed ) ) {
		return 'https://picsum.photos/seed/' . $seed . '/';
	}
	return 'https://picsum.photos/';
	return 'https://placebear.com/';*/
}
/**
 * Filter image URLs for missing files to placekitten.
 *
 * @param array|false $image Array of image data, or boolean false if no image is available.
 *                           [ string: url, int: width, int: height, bool: is_resized ]
 * @param int attachment ID
 * @return array Filtered image.
 */
function localkitten_environment_attachment_image_src( $image, $attachment_id ) {
	if ( ! isset( $image[0] ) || ! is_array( $image ) ) {
		return $image;
	}

	// Return early if we have a file locally.
	$file = get_attached_file( $attachment_id );
	if ( ! empty( $file ) && file_exists( $file ) ) {
		return '';
	}

	// Don't process remote image URLs.
	$image_host = parse_url( $image[0], PHP_URL_HOST );
	$site_host = parse_url( site_url(), PHP_URL_HOST );
	if ( ! ( strpos( $site_host, $image_host ) > 0 ) && ! ( strpos( $image_host, $site_host ) > 0 ) ) {
		return $image;
	}

	$image[0] = localkitten_get_service_url( $attachment_id ) . $image[1] . '/' . $image[2];

	return $image;
}

/**
 * Filters srcset URLs.
 *
 * @param array $sources srcset sources
 * @param array $size_array array of image sizes
 * @param string $image_src source URL
 * @param array $image_meta meta
 * @param int $attachment_id ID of attachment
 * @return array srcset sources with replaced URLs
 */
function localkitten_environment_wp_calculate_image_srcset( array $sources, $size_array, $image_src, $image_meta, $attachment_id ) : array {
	// Return early if this is a locally uploaded asset.
	$file = get_attached_file( $attachment_id );
	if ( ! empty( $file ) && file_exists( $file ) ) {
		return $sources;
	}

	// Return early if we don't have sizes.
	if ( empty( $size_array ) ) {
		return $sources;
	}

	$new_sources = [
		[
			'url'        => localkitten_get_service_url() . $size_array[0] . '/' . $size_array[1],
			'descriptor' => 'w',
			'value'      => $size_array[0],
		],
	];

	return $new_sources;
}

/**
 * Override attachment URLs
 *
 * @param string|mixed $url the URL or falsey
 * @param int $attachment_id ID of the attachment to grab the URL for
 * @return string|mixed the new URL or false
 */
function localkitten_wp_get_attachment_url( $url, $attachment_id ) {
	// Only process images.
	if ( ! wp_attachment_is_image( $attachment_id ) ) {
		return $url;
	}

	// Don't do this is we have the file locally.
	$file = get_attached_file( $attachment_id );
	if ( ! empty( $file ) && file_exists( $file ) ) {
		return $url;
	}

	$image = wp_get_attachment_metadata( $attachment_id, true );

	// We need the width/height to use a placeholder, return early
	// if we don't have them.
	if ( ! $image ) {
		return $url;
	}

	return localkitten_get_service_url( $attachment_id ) . $image['width'] . '/' . $image['height'];
}