Converting external links to open in a new window automagically.

functions.php

<?php

/**
 * Filters given content to add target="_blank" to external links.
 *
 * @see https://developer.wordpress.org/reference/files/wp-includes/html-api/class-wp-html-tag-processor.php/
 * @see https://adamadam.blog/2023/02/16/how-to-modify-html-in-a-php-wordpress-plugin-using-the-new-tag-processor-api/
 * @param string $content Content of the post.
 * @return string
 */
function prefix_external_link_target_blank( $content ): string {

	// Instantiate the processor.
	$processor = new WP_HTML_Tag_Processor( $content );

	// Get the domain of the site without scheme.
	$site_domain = wp_parse_url( site_url(), PHP_URL_HOST );

	// Loop through all the A tags and parse href to see if it's an external link.
	while ( $processor->next_tag( 'A' ) ) {
		$href        = $processor->get_attribute( 'href' );
		$root_domain = wp_parse_url( $href, PHP_URL_HOST );

		// If root domain is null, it's an internal link (no host), skip.
		if ( null === $root_domain ) {
			continue;
		}

		// If the root domain is not the same as the site domain, it's an external link.
		if ( $root_domain !== $site_domain ) {
			$processor->set_attribute( 'target', '_blank' );
			$processor->set_attribute( 'rel', 'nofollow external noopener noreferrer' );
		}
	}

	return $processor->get_updated_html();
}
add_filter( 'the_content', 'prefix_external_link_target_blank', 999 );