diff --git a/src/class-rest-api-endpoints.php b/src/class-rest-api-endpoints.php
index edffd8b5..8a9d0ea4 100644
--- a/src/class-rest-api-endpoints.php
+++ b/src/class-rest-api-endpoints.php
@@ -5,9 +5,12 @@
  * @package wp-newsletter-builder
  */
 
+declare( strict_types=1 );
+
 namespace WP_Newsletter_Builder;
 
-use function WP_Newsletter_Builder\get_byline;
+use WP_Error;
+use WP_REST_Request;
 
 /**
  * Adds additional Endpoints to the REST API.
@@ -25,7 +28,7 @@ public function __construct() {
 	 *
 	 * @return void
 	 */
-	public function register_endpoints() {
+	public function register_endpoints(): void {
 		register_rest_route(
 			'wp-newsletter-builder/v1',
 			'/lists/',
@@ -61,7 +64,7 @@ public function register_endpoints() {
 			]
 		);
 		register_rest_route(
-			'wp-newsletter-builder/v1/',
+			'wp-newsletter-builder/v1',
 			'/status/(?P<post_id>[a-f0-9]+)',
 			[
 				'methods'             => 'GET',
@@ -72,7 +75,7 @@ public function register_endpoints() {
 			]
 		);
 		register_rest_route(
-			'wp-newsletter-builder/v1/',
+			'wp-newsletter-builder/v1',
 			'/subscribe/',
 			[
 				'methods'             => 'POST',
@@ -85,25 +88,27 @@ public function register_endpoints() {
 	/**
 	 * Gets the lists from the Campaign Monitor API.
 	 *
-	 * @return array
+	 * @return WP_Error|array
 	 */
-	public function get_lists() {
+	public function get_lists(): WP_Error|array {
 		if ( ! current_user_can( 'edit_posts' ) ) {
-			// return new \WP_Error( 'rest_forbidden', esc_html__( 'You do not have permission to access this endpoint.', 'wp-newsletter-builder' ), [ 'status' => 401 ] );
+			// phpcs:disable
+			 // TODO return new \WP_Error( 'rest_forbidden', esc_html__( 'You do not have permission to access this endpoint.', 'wp-newsletter-builder' ), [ 'status' => 401 ] );
+			// phpcs:enable
 		}
 		global $newsletter_builder_email_provider;
-		$lists = $newsletter_builder_email_provider->get_lists();
-		return $lists;
+
+		return $newsletter_builder_email_provider->get_lists();
 	}
 
 	/**
 	 * Gets the email types from options.
 	 *
-	 * @return array
+	 * @return WP_Error|array
 	 */
-	public function get_email_types() {
+	public function get_email_types(): WP_Error|array {
 		if ( ! current_user_can( 'edit_posts' ) ) {
-			return new \WP_Error( 'rest_forbidden', esc_html__( 'You do not have permission to access this endpoint.', 'wp-newsletter-builder' ), [ 'status' => 401 ] );
+			return new WP_Error( 'rest_forbidden', esc_html__( 'You do not have permission to access this endpoint.', 'wp-newsletter-builder' ), [ 'status' => 401 ] );
 		}
 		$types_class = new Email_Types();
 		$types       = $types_class->get_email_types();
@@ -119,24 +124,25 @@ public function get_email_types() {
 	/**
 	 * Gets the settings from options.
 	 *
-	 * @return array
+	 * @return WP_Error|array
 	 */
-	public function get_footer_settings() {
+	public function get_footer_settings(): WP_Error|array {
 		if ( ! current_user_can( 'edit_posts' ) ) {
-			return new \WP_Error( 'rest_forbidden', esc_html__( 'You do not have permission to access this endpoint.', 'wp-newsletter-builder' ), [ 'status' => 401 ] );
+			return new WP_Error( 'rest_forbidden', esc_html__( 'You do not have permission to access this endpoint.', 'wp-newsletter-builder' ), [ 'status' => 401 ] );
 		}
 		$settings        = new Settings();
-		$footer_settings = $settings->get_footer_settings();
-		return $footer_settings;
+
+		return $settings->get_footer_settings();
 	}
 
 	/**
 	 * Gets the status for a newsletter.
 	 *
-	 * @param \WP_REST_Request $request The request object.
+	 * @param WP_REST_Request $request The request object.
+	 *
 	 * @return array
 	 */
-	public function get_status( $request ) {
+	public function get_status( WP_REST_Request $request ): array {
 		$post_id = $request->get_param( 'post_id' );
 		if ( empty( $post_id ) ) {
 			return [];
@@ -194,10 +200,11 @@ public function get_status( $request ) {
 	/**
 	 * Subscribes a user to a list.
 	 *
-	 * @param \WP_Rest_Request $request The request object.
+	 * @param WP_Rest_Request $request The request object.
+	 *
 	 * @return array
 	 */
-	public function subscribe( $request ) {
+	public function subscribe( WP_REST_Request $request ): array {
 		$email = $request->get_param( 'email' );
 		if ( empty( $email ) ) {
 			return [
diff --git a/src/class-wp-newsletter-builder.php b/src/class-wp-newsletter-builder.php
index a5c885c4..5929cac1 100644
--- a/src/class-wp-newsletter-builder.php
+++ b/src/class-wp-newsletter-builder.php
@@ -7,6 +7,8 @@
 
 namespace WP_Newsletter_Builder;
 
+use WP_Post;
+
 /**
  * Example Plugin
  */
@@ -97,7 +99,7 @@ public function include_template( $template ) {
 		$local_path = WP_NEWSLETTER_BUILDER_DIR . '/single-nb_newsletter.php';
 
 		if (
-			$post instanceof \WP_Post
+			$post instanceof WP_Post
 			&& is_singular( 'nb_newsletter' )
 			&& file_exists( $local_path )
 			&& 0 === validate_file( $local_path )
@@ -111,18 +113,20 @@ public function include_template( $template ) {
 	/**
 	 * Sends the newsletter when the newsletter post is published.
 	 *
-	 * @param int      $post_id The post id.
-	 * @param \WP_Post $post The post.
-	 * @param bool     $update Whether this is an update.
-	 * @param \WP_Post $post_before The post before the update.
+	 * @param int $post_id The post id.
+	 * @param WP_Post $post The post.
+	 * @param bool $update Whether this is an update.
+	 * @param WP_Post|null $post_before The post before the update.
 	 */
-	public function on_newsletter_after_insert_post( $post_id, $post, $update, $post_before ): void {
+	public function on_newsletter_after_insert_post( int $post_id, WP_Post $post, bool $update, ?WP_Post $post_before ): void {
 		if ( 'nb_newsletter' !== $post->post_type ) {
 			return;
 		}
 		$new_status = $post->post_status;
-		$old_status = $post_before->post_status;
-		if ( $new_status === $old_status || 'publish' !== $new_status ) {
+		if (
+			$new_status === $post_before?->post_status
+			|| 'publish' !== $new_status
+		) {
 			return;
 		}
 		$this->do_send( $post->ID );