Prep Plugin for release on WordPress.org

Escape everything that should be escaped.
Add nonce checks where needed.
Sanitize all inputs.
Apply Code style changes across the codebase.
Correct many deprecation notices.
Optimize load order of many filters.

inc/checkout/class-checkout.php

@@ -1960,7 +1960,7 @@ class Checkout {
 				/*
 				 * Redirect go burrr!
 				 */
-				wp_redirect($redirect_url);
+				wp_safe_redirect($redirect_url);
 
 				exit;
 			}
@@ -2002,7 +2002,7 @@ class Checkout {
 				*/
 				$next_step = $this->get_next_step_name();
 
-				wp_redirect(add_query_arg('step', $next_step));
+				wp_safe_redirect(add_query_arg('step', $next_step));
 
 				exit;
 			}
@@ -2165,7 +2165,7 @@ class Checkout {
 				);
 			}
 
-			wp_redirect($redirect_url);
+			wp_safe_redirect($redirect_url);
 
 			exit;
 		} catch (\Throwable $e) {
@@ -2286,12 +2286,12 @@ class Checkout {
 	 * @since 2.0.0
 	 *
 	 * @param string $key Key to retrieve the value for.
-	 * @param mixed  $default The default value to return, when nothing is found.
+	 * @param mixed  $default_value The default value to return, when nothing is found.
 	 * @return mixed
 	 */
-	public function request_or_session($key, $default = false) {
+	public function request_or_session($key, $default_value = false) {
 
-		$value = $default;
+		$value = $default_value;
 
 		if (null !== $this->session) {
 			$session = $this->session->get('signup');