WordPress post link replacement for use on headless installs.

headless-link-replacement.php

<?php
/**
 * Customize the preview button in the WordPress admin to point to the headless frontend.
 *
 * @param  string  $link WordPress preview link.
 * @param  WP_Post $post Current post object.
 * @return string        The headless WordPress preview link.
 */
function set_headless_preview_link( string $link, \WP_Post $post ) {
	$frontend_url = 'https://my-headless-frontend.test';
	$slug         = strlen( $post->post_name ) > 0 ? $post->post_name : sanitize_title( $post->post_title );

	// Get GraphQL single name for current post type (with regular post type as fallback).
	$post_type = get_post_type_object( $post->post_type )->graphql_single_name ?? $post->post_type;

	// Preview link will have format: <domain>/api/preview?name=<slug>&id=<post-id>&post_type=<post-type>&token=<preview-token>.
	return add_query_arg(
		[
			'name'      => $slug,
			'id'        => $post->ID,
			'post_type' => $post_type,
      // Preview secret token is used to authenticate frontend requests for post previews.
      // This line assumes the token is set via `wp-config.php`.
			'token'     => defined( 'PREVIEW_SECRET_TOKEN' ) ? PREVIEW_SECRET_TOKEN : '',
		],
		"{$frontend_url}/api/preview"
	);
}
add_filter( 'preview_post_link', 'set_headless_preview_link', 10, 2 );

/**
 * Customize WP home URL to point to frontend.
 *
 * @param  string $url    Complete home URL, including path.
 * @param  string $path   Path relative to home URL.
 * @param  string $scheme Context for home URL.
 * @return string         Frontend home URL.
 */
function set_headless_home_url( string $url, string $path, $scheme = null ) {
	// Don't redirect REST requests.
	if ( 'rest' === $scheme ) {
		return $url;
	}

	// Don't redirect unless in WP admin.
	if ( ! is_admin() ) {
		return $url;
	}

	$frontend_url = 'https://my-headless-frontend.test';

	if ( ! $path ) {
		return $frontend_url;
	}

	// Remove excess slash from beginning of path.
	$path = ltrim( $path, '/' );

	return "{$frontend_url}/{$path}";
}
add_filter( 'home_url', 'set_headless_home_url', 10, 3 );

/**
 * Customize the REST link to point to the headless frontend.
 *
 * @param  WP_REST_Response $response Response object.
 * @param  WP_Post          $post     Current post object.
 * @return WP_REST_Response           Response object.
 */
function set_headless_rest_link( \WP_REST_Response $response, \WP_Post $post ) {
	if ( 'draft' === $post->post_status ) {

		// Manually call preview filter for draft posts.
		$response->data['link'] = get_preview_post_link( $post );
	} elseif ( 'publish' === $post->post_status ) {

		// Override view link for published posts.
		$frontend_url = 'https://my-headless-frontend.test';
    $permalink    = get_permalink( $post );
    $site_url     = get_site_url();

    // Replace site URL if present.
    if ( false !== stristr( $permalink, $site_url ) ) {
      $permalink = str_ireplace( $site_url, $frontend_url, $permalink );
    }

    // Replace link with new permalink.
    $response->data['link'] = $permalink;
	}

	return $response;
}
add_filter( 'rest_prepare_page', 'set_headless_rest_link', 10, 2 );
add_filter( 'rest_prepare_post', 'set_headless_rest_link', 10, 2 );
// For use with custom post types, see https://developer.wordpress.org/reference/hooks/rest_prepare_this-post_type/.

/**
 * Override links within post content on save to point to frontend.
 *
 * @param int $post_id Post ID.
 */
function override_post_links( $post_id ) {

	// Unhook function to avoid infinite loop.
	remove_action( 'save_post', 'override_post_links' );

	$post = get_post( $post_id );

	if ( ! $post ) {
		return;
	}

	$post_content = $post->post_content;
	$backend_url  = get_site_url();

	// Check if post content contains backend WordPress links.
	if ( false === stripos( $post_content, $backend_url ) ) {
		return;
	}

	$frontend_url     = 'https://my-headless-frontend.test';
	$new_post_content = $post_content;

	// Replace backend URL with frontend URL.
	$new_post_content = str_ireplace( $backend_url, $frontend_url, $post_content );

	// Revert media links.
  // Media links on frontend should point to location on WordPress install.
	$upload_dir       = wp_upload_dir();
	$upload_dir       = str_ireplace( $backend_url, '', $upload_dir['baseurl'] );
	$new_post_content = str_ireplace( "{$frontend_url}{$upload_dir}", "{$backend_url}{$upload_dir}", $new_post_content );

	// Revert plugin links.
  // Links to plugin files on frontend should point to location on WordPress install.
	$plugin_dir       = defined( 'WP_PLUGIN_URL' ) ? WP_PLUGIN_URL : '/wp-content/plugins';
	$plugin_dir       = str_ireplace( $backend_url, '', $plugin_dir );
	$new_post_content = str_ireplace( "{$frontend_url}{$plugin_dir}", "{$backend_url}{$plugin_dir}", $new_post_content );

	// Save post.
	wp_update_post(
		[
			'ID'           => $post_id,
			'post_content' => wp_slash( $new_post_content ),
		]
	);

	// Re-hook function.
	add_action( 'save_post', 'override_post_links' );
}
add_action( 'save_post', 'override_post_links' );