diff --git a/src/admin/class-admin.php b/src/admin/class-admin.php
index 5c19646..49ec24f 100644
--- a/src/admin/class-admin.php
+++ b/src/admin/class-admin.php
@@ -115,12 +115,12 @@ class Admin {
 			array(),
 			( PRODUCTION_ENV ) ? $this->version : filemtime(
 				plugin_dir_path(
-					 __FILE__ 
+					__FILE__
 				) . 'css/settings.css'
 			),
 			'all'
 		);
-		
+
 	}
 
 	/**
@@ -136,7 +136,7 @@ class Admin {
 			array(),
 			( PRODUCTION_ENV ) ? $this->version : filemtime(
 				plugin_dir_path(
-					__FILE__ 
+					__FILE__
 				) . 'js/wysiwyg-editor.js'
 			),
 			false
diff --git a/src/admin/class-wysiwyg.php b/src/admin/class-wysiwyg.php
index bc74448..55069dc 100644
--- a/src/admin/class-wysiwyg.php
+++ b/src/admin/class-wysiwyg.php
@@ -12,7 +12,7 @@
  * @since  2.8.0  Rename file from `wysiwyg.php` to `class-footnotes-wysiwyg.php`,
  *                              move from `class/` sub-directory to `admin/`.
  */
- 
+
 declare(strict_types=1);
 
 namespace footnotes\admin;
diff --git a/src/admin/layout/class-engine.php b/src/admin/layout/class-engine.php
index 1a960c2..c03d6e1 100644
--- a/src/admin/layout/class-engine.php
+++ b/src/admin/layout/class-engine.php
@@ -583,7 +583,7 @@ abstract class Engine {
 	 * @since  1.5.0
 	 * @todo  Refactor HTML generation.
 	 */
-	protected function add_num_box( string  $p_str_setting_name, int $p_in_min, int $p_int_max, bool $p_bool_deci = false ): string {
+	protected function add_num_box( string $p_str_setting_name, int $p_in_min, int $p_int_max, bool $p_bool_deci = false ): string {
 		// Collect data for given settings field.
 		$l_arr_data = $this->load_setting( $p_str_setting_name );
 
diff --git a/src/admin/layout/class-settings.php b/src/admin/layout/class-settings.php
index 4b5359b..53ec42d 100644
--- a/src/admin/layout/class-settings.php
+++ b/src/admin/layout/class-settings.php
@@ -1231,8 +1231,8 @@ class Settings extends Engine {
 	 * @todo Review in light of admin/public split.
 	 */
 	public function help(): void {
-	  $footnotes = new General\General($this->plugin_name, "foo");
-		
+		$footnotes = new General\General( $this->plugin_name, 'foo' );
+
 		// Load footnotes starting and end tag.
 		$l_arr_footnote_starting_tag = $this->load_setting( \footnotes\includes\Settings::C_STR_FOOTNOTES_SHORT_CODE_START );
 		$l_arr_footnote_ending_tag   = $this->load_setting( \footnotes\includes\Settings::C_STR_FOOTNOTES_SHORT_CODE_END );
diff --git a/src/footnotes.php b/src/footnotes.php
index e929969..0718185 100755
--- a/src/footnotes.php
+++ b/src/footnotes.php
@@ -25,6 +25,8 @@
  * License URI: https://www.gnu.org/licenses/gpl-3.0.html
  */
 
+declare(strict_types=1);
+
 namespace footnotes;
 
 // If this file is called directly, abort.
@@ -90,12 +92,13 @@ function deactivate_footnotes(): void {
 
 /*
  * TODO: currently these throw an error:
- * Uncaught TypeError: call_user_func_array(): Argument #1 ($function) must be 
- * a valid callback, function "deactivate_footnotes" not found or invalid 
+ * Uncaught TypeError: call_user_func_array(): Argument #1 ($function) must be
+ * a valid callback, function "deactivate_footnotes" not found or invalid
  * function name in /srv/www/wordpress-one/public_html/wp-includes/class-wp-hook.php:292
+ *
+ * register_activation_hook( __FILE__, 'activate_footnotes' );
+ * register_deactivation_hook( __FILE__, 'deactivate_footnotes' );
  */
-//register_activation_hook( __FILE__, 'activate_footnotes' );
-//register_deactivation_hook( __FILE__, 'deactivate_footnotes' );
 
 /**
  * The core plugin class that defines internationalization, admin-specific and
diff --git a/src/includes/class-activator.php b/src/includes/class-activator.php
index c11e313..63a7d1a 100644
--- a/src/includes/class-activator.php
+++ b/src/includes/class-activator.php
@@ -25,6 +25,8 @@
  * @since 2.8.0
  */
 
+declare(strict_types=1);
+
 namespace footnotes\includes;
 
 /**
diff --git a/src/includes/class-config.php b/src/includes/class-config.php
index 72620c9..8f93d29 100644
--- a/src/includes/class-config.php
+++ b/src/includes/class-config.php
@@ -29,6 +29,8 @@
  * @deprecated
  */
 
+declare(strict_types=1);
+
 namespace footnotes\includes;
 
 /**
diff --git a/src/includes/class-convert.php b/src/includes/class-convert.php
index 3e70b87..0d142c8 100644
--- a/src/includes/class-convert.php
+++ b/src/includes/class-convert.php
@@ -8,6 +8,8 @@
  *                              rename `class/` sub-directory to `includes/`.
  */
 
+declare(strict_types=1);
+
 namespace footnotes\includes;
 
 /**
@@ -40,7 +42,7 @@ class Convert {
 				return self::to_arabic_leading( $p_int_index );
 			case 'arabic_plain':
 			default:
-				return $p_int_index;
+				return (string) $p_int_index;
 		}
 	}
 
diff --git a/src/includes/class-core.php b/src/includes/class-core.php
index 7a64fe2..69a02a9 100644
--- a/src/includes/class-core.php
+++ b/src/includes/class-core.php
@@ -27,6 +27,8 @@
  *              Renamed parent `class/` directory to `includes/`.
  */
 
+declare(strict_types=1);
+
 namespace footnotes\includes;
 
 use footnotes\general as General;
diff --git a/src/includes/class-deactivator.php b/src/includes/class-deactivator.php
index 8c2aa9a..0f13e6f 100644
--- a/src/includes/class-deactivator.php
+++ b/src/includes/class-deactivator.php
@@ -6,6 +6,8 @@
  * @since 2.8.0
  */
 
+declare(strict_types=1);
+
 namespace footnotes\includes;
 
 /**
diff --git a/src/includes/class-i18n.php b/src/includes/class-i18n.php
index 4d7a2fd..626596b 100644
--- a/src/includes/class-i18n.php
+++ b/src/includes/class-i18n.php
@@ -8,6 +8,8 @@
  *                              rename `class/` sub-directory to `includes/`.
  */
 
+declare(strict_types=1);
+
 namespace footnotes\includes;
 
 require_once plugin_dir_path( dirname( __FILE__ ) ) . 'includes/class-config.php';
diff --git a/src/includes/class-loader.php b/src/includes/class-loader.php
index 62915b6..fe93898 100644
--- a/src/includes/class-loader.php
+++ b/src/includes/class-loader.php
@@ -7,6 +7,8 @@
  * @since 2.8.0
  */
 
+declare(strict_types=1);
+
 namespace footnotes\includes;
 
 /**
diff --git a/src/includes/class-settings.php b/src/includes/class-settings.php
index 8d31708..7a279fd 100644
--- a/src/includes/class-settings.php
+++ b/src/includes/class-settings.php
@@ -8,6 +8,8 @@
  *              Renamed parent `class/` directory to `includes/`.
  */
 
+declare(strict_types=1);
+
 namespace footnotes\includes;
 
 /**
diff --git a/src/includes/class-template.php b/src/includes/class-template.php
index 86f8edb..a05d810 100644
--- a/src/includes/class-template.php
+++ b/src/includes/class-template.php
@@ -11,6 +11,8 @@
  *                              rename `class/` sub-directory to `includes/`.
  */
 
+declare(strict_types=1);
+
 namespace footnotes\includes;
 
 /**
@@ -120,7 +122,7 @@ class Template {
 		}
 		// Iterate through each placeholder and replace it with its value.
 		foreach ( $p_arr_placeholders as $l_str_placeholder => $l_str_value ) {
-			$this->a_str_replaced_content = str_replace( '[[' . $l_str_placeholder . ']]', $l_str_value, $this->a_str_replaced_content );
+			$this->a_str_replaced_content = str_replace( '[[' . $l_str_placeholder . ']]', (string) $l_str_value, $this->a_str_replaced_content );
 		}
 		// Success.
 		return true;
diff --git a/src/public/class-general.php b/src/public/class-general.php
index 6228099..fd1d8aa 100644
--- a/src/public/class-general.php
+++ b/src/public/class-general.php
@@ -6,6 +6,8 @@
  * @since 2.8.0
  */
 
+declare(strict_types=1);
+
 namespace footnotes\general;
 
 use footnotes\includes as Includes;
@@ -29,7 +31,7 @@ class General {
 	 * @access  private
 	 * @var  string  $plugin_name  The ID of this plugin.
 	 */
-	private $plugin_name;
+	private string $plugin_name;
 
 	/**
 	 * The version of this plugin.
@@ -39,7 +41,7 @@ class General {
 	 * @access  private
 	 * @var  string  $version  The current version of this plugin.
 	 */
-	private $version;
+	private string $version;
 
 	/**
 	 * The reference container widget.
@@ -48,17 +50,18 @@ class General {
 	 *
 	 * @var  Widget\Reference_Container  $reference_container_widget  The reference container widget
 	 */
-	private $reference_container_widget;
+	private Widget\Reference_Container $reference_container_widget;
 
 	/**
 	 * The footnote parser.
 	 *
 	 * @since  1.5.0
 	 * @since  2.8.0  Moved from {@see Footnotes} to {@see Includes\Public}.
+	 * @todo Review null init.
 	 *
 	 * @var  Parser  $task  The Plugin task.
 	 */
-	public $a_obj_task = null;
+	public ?Parser $a_obj_task = null;
 
 	/**
 	 * Flag for using tooltips.
@@ -107,7 +110,7 @@ class General {
 	 * @param  string $plugin_name  The name of this plugin.
 	 * @param  string $version  The version of this plugin.
 	 */
-	public function __construct( $plugin_name, $version ) {
+	public function __construct( string $plugin_name, string $version ) {
 
 		$this->plugin_name = $plugin_name;
 		$this->version     = $version;
@@ -132,7 +135,7 @@ class General {
 	 *
 	 * @since  2.8.0
 	 */
-	private function load_dependencies() {
+	private function load_dependencies(): void {
 		// TODO: neaten up and document once placements and names are settled.
 		require_once plugin_dir_path( dirname( __FILE__ ) ) . 'includes/class-config.php';
 		require_once plugin_dir_path( dirname( __FILE__ ) ) . 'includes/class-settings.php';
@@ -156,7 +159,7 @@ class General {
 	 * @since  2.5.5  Change stylesheet schema.
 	 * @since  2.8.0  Moved from {@see Footnotes} to {@see Includes\Public}.
 	 */
-	public function enqueue_styles() {
+	public function enqueue_styles(): void {
 		if ( PRODUCTION_ENV ) {
 			// Set tooltip mode for use in stylesheet name.
 			if ( self::$a_bool_tooltips_enabled ) {
@@ -204,20 +207,20 @@ class General {
 				array(),
 				( PRODUCTION_ENV ) ? $this->version : filemtime(
 					plugin_dir_path(
-						 __FILE__ 
+						__FILE__
 					) . "css/footnotes-{$l_str_tooltip_mode_short}brpl{$l_str_layout_mode}.min.css"
 				),
 				'all'
 			);
 		} else {
-			foreach (array('amp-tooltips', 'common', 'layout-entry-content', 'layout-main-content', 'layout-reference-container', 'tooltips', 'tooltips-alternative') as $val) {
+			foreach ( array( 'amp-tooltips', 'common', 'layout-entry-content', 'layout-main-content', 'layout-reference-container', 'tooltips', 'tooltips-alternative' ) as $val ) {
 				wp_enqueue_style(
 					"footnotes-$val",
 					plugin_dir_url( __FILE__ ) . "css/dev-$val.css",
 					array(),
 					filemtime(
 						plugin_dir_path(
-							 __FILE__ 
+							__FILE__
 						) . "css/dev-$val.css"
 					),
 					'all'
@@ -235,7 +238,7 @@ class General {
 	 * @since  2.5.6  Add jQuery dependency.
 	 * @since  2.8.0  Moved from {@see Footnotes} to {@see Includes\Public}.
 	 */
-	public function enqueue_scripts() {
+	public function enqueue_scripts(): void {
 		/*
 		 * Enqueues the jQuery library registered by WordPress.
 		 *
@@ -284,7 +287,7 @@ class General {
 	 * @since  1.5.0
 	 * @since  2.8.0  Moved from {@see Footnotes} to {@see Includes\Public}.
 	 */
-	public function register_widgets() {
+	public function register_widgets(): void {
 		register_widget( $this->reference_container_widget );
 	}
 }
diff --git a/src/public/class-parser.php b/src/public/class-parser.php
index 4b55203..194a4e7 100644
--- a/src/public/class-parser.php
+++ b/src/public/class-parser.php
@@ -9,6 +9,8 @@
  *                              move from `class/` sub-directory to `public/`.
  */
 
+declare(strict_types=1);
+
 namespace footnotes\general;
 
 use footnotes\includes as Includes;
@@ -284,7 +286,7 @@ class Parser {
 	 * @since 2.1.0  Remove @see 'the_post' support.
 	 * @todo Move to {@see General}.
 	 */
-	public function register_hooks() {
+	public function register_hooks(): void {
 		// Get values from settings.
 		$l_int_the_title_priority    = (int) Includes\Settings::instance()->get( \footnotes\includes\Settings::C_INT_EXPERT_LOOKUP_THE_TITLE_PRIORITY_LEVEL );
 		$l_int_the_content_priority  = (int) Includes\Settings::instance()->get( \footnotes\includes\Settings::C_INT_EXPERT_LOOKUP_THE_CONTENT_PRIORITY_LEVEL );
@@ -431,7 +433,7 @@ class Parser {
 	 * @since 1.5.0
 	 * @todo Refactor to enqueue stylesheets properly in {@see General}.
 	 */
-	public function footnotes_output_head() {
+	public function footnotes_output_head(): void {
 
 		// Insert start tag without switching out of PHP.
 		echo "\r\n<style type=\"text/css\" media=\"all\">\r\n";
@@ -746,7 +748,7 @@ class Parser {
 	 *
 	 * @since 1.5.0
 	 */
-	public function footnotes_output_footer() {
+	public function footnotes_output_footer(): void {
 		if ( 'footer' === Includes\Settings::instance()->get( \footnotes\includes\Settings::C_STR_REFERENCE_CONTAINER_POSITION ) ) {
 			echo $this->reference_container();
 		}
@@ -805,7 +807,7 @@ class Parser {
 	 * @param string $p_str_content  Title.
 	 * @return string  $p_str_content  Title with replaced footnotes.
 	 */
-	public function footnotes_in_title( $p_str_content ) {
+	public function footnotes_in_title( string $p_str_content ): string {
 		// Appends the reference container if set to "post_end".
 		return $this->exec( $p_str_content, false );
 	}
@@ -818,7 +820,7 @@ class Parser {
 	 * @param string $p_str_content  Page/Post content.
 	 * @return string  $p_str_content  Content with replaced footnotes.
 	 */
-	public function footnotes_in_content( $p_str_content ) {
+	public function footnotes_in_content( string $p_str_content ): string {
 
 		$l_str_ref_container_position            = Includes\Settings::instance()->get( \footnotes\includes\Settings::C_STR_REFERENCE_CONTAINER_POSITION );
 		$l_str_footnote_section_shortcode        = Includes\Settings::instance()->get( \footnotes\includes\Settings::C_STR_FOOTNOTE_SECTION_SHORTCODE );
@@ -864,7 +866,7 @@ class Parser {
 	 * @param string $p_str_excerpt  Excerpt content.
 	 * @return string  $p_str_excerpt  Processed or new excerpt.
 	 */
-	public function footnotes_in_excerpt( $p_str_excerpt ) {
+	public function footnotes_in_excerpt( string $p_str_excerpt ): string {
 		$l_str_excerpt_mode = Includes\Settings::instance()->get( \footnotes\includes\Settings::C_STR_FOOTNOTES_IN_EXCERPT );
 
 		if ( 'yes' === $l_str_excerpt_mode ) {
@@ -891,7 +893,7 @@ class Parser {
 	 * @param string $p_str_content  The post.
 	 * @return string  $p_str_content  An excerpt of the post.
 	 */
-	public function generate_excerpt( $p_str_content ) {
+	public function generate_excerpt( string $p_str_content ): string {
 
 		// Discard existing excerpt and start on the basis of the post.
 		$p_str_content = get_the_content( get_the_id() );
@@ -936,7 +938,7 @@ class Parser {
 	 * @param string $p_str_content  The post.
 	 * @return string  $p_str_content  An excerpt of the post.
 	 */
-	public function generate_excerpt_with_footnotes( $p_str_content ) {
+	public function generate_excerpt_with_footnotes( string $p_str_content ): string {
 
 		// Discard existing excerpt and start on the basis of the post.
 		$p_str_content = get_the_content( get_the_id() );
@@ -1037,7 +1039,7 @@ class Parser {
 	 * @param string $p_str_content  Widget content.
 	 * @return string  $p_str_content  Content with replaced footnotes.
 	 */
-	public function footnotes_in_widget_title( $p_str_content ) {
+	public function footnotes_in_widget_title( string $p_str_content ): string {
 		// Appends the reference container if set to "post_end".
 		return $this->exec( $p_str_content, false );
 	}
@@ -1050,7 +1052,7 @@ class Parser {
 	 * @param string $p_str_content  Widget content.
 	 * @return string  $p_str_content  Content with replaced footnotes.
 	 */
-	public function footnotes_in_widget_text( $p_str_content ) {
+	public function footnotes_in_widget_text( string $p_str_content ): string {
 		// phpcs:disable WordPress.PHP.YodaConditions.NotYoda
 		// Appends the reference container if set to "post_end".
 		return $this->exec( $p_str_content, 'post_end' === Includes\Settings::instance()->get( \footnotes\includes\Settings::C_STR_REFERENCE_CONTAINER_POSITION ) );
@@ -1067,7 +1069,7 @@ class Parser {
 	 * @param bool   $p_bool_hide_footnotes_text  Hide footnotes found in the string.
 	 * @return string
 	 */
-	public function exec( $p_str_content, $p_bool_output_references = false, $p_bool_hide_footnotes_text = false ) {
+	public function exec( string $p_str_content, bool $p_bool_output_references = false, bool $p_bool_hide_footnotes_text = false ): string {
 
 		// Process content.
 		$p_str_content = $this->search( $p_str_content, $p_bool_hide_footnotes_text );
@@ -1122,7 +1124,7 @@ class Parser {
 	 *
 	 * @param string $p_str_content  The footnote, including delimiters.
 	 */
-	public function unify_delimiters( $p_str_content ) {
+	public function unify_delimiters( string $p_str_content ): string {
 
 		// Get footnotes start and end tag short codes.
 		$l_str_starting_tag = Includes\Settings::instance()->get( \footnotes\includes\Settings::C_STR_FOOTNOTES_SHORT_CODE_START );
@@ -1184,12 +1186,13 @@ class Parser {
 	 *
 	 * @since 1.5.0
 	 * @todo Refactor to parse DOM rather than using RegEx.
+	 * @todo Decompose.
 	 *
 	 * @param string $p_str_content  Any content to be parsed for footnotes.
 	 * @param bool   $p_bool_hide_footnotes_text  Hide footnotes found in the string.
 	 * @return string
 	 */
-	public function search( $p_str_content, $p_bool_hide_footnotes_text ) {
+	public function search( string $p_str_content, bool $p_bool_hide_footnotes_text ): string {
 
 		// Get footnote delimiter shortcodes and unify them.
 		$p_str_content = self::unify_delimiters( $p_str_content );
@@ -1701,7 +1704,7 @@ class Parser {
 	 *
 	 * @return string
 	 */
-	public function reference_container() {
+	public function reference_container(): string {
 
 		// No footnotes have been replaced on this page.
 		if ( empty( self::$a_arr_footnotes ) ) {
diff --git a/src/public/widget/class-base.php b/src/public/widget/class-base.php
index 3a5c9b9..324f356 100644
--- a/src/public/widget/class-base.php
+++ b/src/public/widget/class-base.php
@@ -10,6 +10,8 @@
  * @since 1.5.0
  */
 
+declare(strict_types=1);
+
 namespace footnotes\general\Widget;
 
 use footnotes\includes as Includes;
@@ -36,7 +38,7 @@ abstract class Base extends \WP_Widget {
 	 *
 	 * @return  string
 	 */
-	abstract protected function get_id();
+	abstract protected function get_id(): string;
 
 	/**
 	 * Returns the Public name of child Widget to be displayed in the Configuration page.
@@ -46,7 +48,7 @@ abstract class Base extends \WP_Widget {
 	 *
 	 * @return  string
 	 */
-	abstract protected function get_name();
+	abstract protected function get_name(): string;
 
 	/**
 	 * Returns the Description of the child widget.
@@ -56,7 +58,7 @@ abstract class Base extends \WP_Widget {
 	 *
 	 * @return  string
 	 */
-	abstract protected function get_description();
+	abstract protected function get_description(): string;
 
 	/**
 	 * Returns the width of the Widget. Default width is 250 pixel.
@@ -65,7 +67,7 @@ abstract class Base extends \WP_Widget {
 	 *
 	 * @return  int
 	 */
-	protected function get_widget_width() {
+	protected function get_widget_width(): int {
 		return 250;
 	}
 
diff --git a/src/public/widget/class-reference-container.php b/src/public/widget/class-reference-container.php
index c051a96..52061e6 100644
--- a/src/public/widget/class-reference-container.php
+++ b/src/public/widget/class-reference-container.php
@@ -10,6 +10,9 @@
  * @since  1.5.0
  */
 
+// TODO: Disabled pending WPWidget AP review.
+/* declare(strict_types=1); */
+
 namespace footnotes\general\Widget;
 
 use footnotes\includes as Includes;
@@ -34,7 +37,7 @@ class Reference_Container extends Base {
 	 * @see  Includes\Footnotes::$plugin_name
 	 * @var  string  $plugin_name  The ID of this plugin.
 	 */
-	private $plugin_name;
+	private string $plugin_name;
 
 	/**
 	 * Initialize the class and set its properties.
@@ -43,9 +46,9 @@ class Reference_Container extends Base {
 	 *
 	 * @param  string $plugin_name  The name of this plugin.
 	 */
-	public function __construct( $plugin_name ) {
-		parent::__construct();
+	public function __construct( string $plugin_name ) {
 		$this->plugin_name = $plugin_name;
+		parent::__construct();
 	}
 
 	/**
@@ -56,7 +59,7 @@ class Reference_Container extends Base {
 	 *
 	 * @return  string
 	 */
-	protected function get_id() {
+	protected function get_id(): string {
 		return 'footnotes_widget';
 	}
 
@@ -68,7 +71,7 @@ class Reference_Container extends Base {
 	 *
 	 * @return  string
 	 */
-	protected function get_name() {
+	protected function get_name(): string {
 		return $this->plugin_name;
 	}
 
@@ -80,7 +83,7 @@ class Reference_Container extends Base {
 	 *
 	 * @return  string
 	 */
-	protected function get_description() {
+	protected function get_description(): string {
 		return __( 'The widget defines the position of the reference container if set to &ldquo;widget area&rdquo;.', 'footnotes' );
 	}
 
diff --git a/src/uninstall.php b/src/uninstall.php
index 092260d..4c8e3f8 100644
--- a/src/uninstall.php
+++ b/src/uninstall.php
@@ -16,6 +16,8 @@
  * @since  2.8.0
  */
 
+declare(strict_types=1);
+
 // If uninstall not called from WordPress, then exit.
 if ( ! defined( 'WP_UNINSTALL_PLUGIN' ) ) {
 	exit;