WordPress 把外站圖片自動下載進自己的媒體庫

<img src="https://old.example.com/uploads/photo.jpg">

// 發佈或更新文章時，把指定網域的外站圖片下載到媒體庫，並換成自己網站的網址
add_action( 'save_post_post', 'zw_import_external_images', 20, 2 );

function zw_import_external_images( $post_id, $post ) {
	if ( wp_is_post_revision( $post_id ) || wp_is_post_autosave( $post_id ) || 'publish' !== $post->post_status ) {
		return;
	}
	// 只搬這些網域的圖片：你自己的舊網站、舊圖床
	$allowed_hosts = array( 'old.example.com' );

	if ( ! preg_match_all( '/<img[^>]+src=["\']([^"\']+)["\']/i', $post->post_content, $matches ) ) {
		return;
	}
	require_once ABSPATH . 'wp-admin/includes/media.php';
	require_once ABSPATH . 'wp-admin/includes/file.php';
	require_once ABSPATH . 'wp-admin/includes/image.php';

	$content = $post->post_content;
	foreach ( array_unique( $matches[1] ) as $url ) {
		$host = wp_parse_url( $url, PHP_URL_HOST );
		if ( ! $host || ! in_array( $host, $allowed_hosts, true ) ) {
			continue;
		}
		// 同一張圖下載過就直接用，不重複下載
		$existing = get_posts( array(
			'post_type'   => 'attachment',
			'post_status' => 'inherit',
			'meta_key'    => '_zw_source_url',
			'meta_value'  => $url,
			'fields'      => 'ids',
			'numberposts' => 1,
		) );
		if ( $existing ) {
			$attachment_id = $existing[0];
		} else {
			$attachment_id = media_sideload_image( $url, $post_id, null, 'id' );
			if ( is_wp_error( $attachment_id ) ) {
				continue; // 下載失敗就保留原本的網址
			}
			update_post_meta( $attachment_id, '_zw_source_url', $url );
		}
		$content = str_replace( $url, wp_get_attachment_url( $attachment_id ), $content );
	}

	if ( $content !== $post->post_content ) {
		// 先拿掉自己再更新，不然更新文章又會觸發這段，變成無窮迴圈
		remove_action( 'save_post_post', 'zw_import_external_images', 20 );
		wp_update_post( array( 'ID' => $post_id, 'post_content' => $content ) );
		add_action( 'save_post_post', 'zw_import_external_images', 20, 2 );
	}
}