diff --git a/tasks/phpstan/bootstrap.php b/tasks/phpstan/bootstrap.php
index 8e99a40a30..9e09aa8ed4 100644
--- a/tasks/phpstan/bootstrap.php
+++ b/tasks/phpstan/bootstrap.php
@@ -1,21 +1,11 @@
 <?php
 
 // Constants
-define('ABSPATH', getenv('WP_ROOT') . '/');
 define('WPINC', 'wp-includes');
-define('WP_DEBUG', false);
 define('WP_LANG_DIR', 'wp-content/languages');
 define('WP_PLUGIN_DIR', 'wp-content/plugins');
 define('WP_MEMORY_LIMIT', 268435456);
 define('WP_MAX_MEMORY_LIMIT', 268435456);
-define('ARRAY_A', 'ARRAY_A');
-define('OBJECT', 'OBJECT');
-define('MINUTE_IN_SECONDS', 60);
-define('HOUR_IN_SECONDS', 60 * MINUTE_IN_SECONDS);
-define('DAY_IN_SECONDS', 24 * HOUR_IN_SECONDS);
-define('WEEK_IN_SECONDS', 7 * DAY_IN_SECONDS);
-define('MONTH_IN_SECONDS', 30 * DAY_IN_SECONDS);
-define('YEAR_IN_SECONDS', 365 * DAY_IN_SECONDS);
 define('MAILPOET_VERSION', '1.0.0');
 define('DB_HOST', 'localhost');
 define('DB_NAME', 'wordpress');
@@ -30,8 +20,8 @@ $dbConfig->defineTables();
 MailPoet\Config\Env::$dbPrefix = 'wp_';
 
 // Classes needed in runtime
-MailPoet\Mailer\WordPress\PHPMailerLoader::load();
-require_once(ABSPATH . 'wp-includes/class-wp-widget.php'); // We extend this class it is need in runtime
+class_alias(\PHPMailer\PHPMailer\PHPMailer::class, 'PHPMailer');
+class_alias(\PHPMailer\PHPMailer\Exception::class, 'phpmailerException');
 // Load tracy
 $tracyPath = __DIR__ . '/../../tools/vendor/tracy.phar';
 require_once($tracyPath);
diff --git a/tasks/phpstan/phpstan-baseline-integration-tests.neon b/tasks/phpstan/phpstan-baseline-integration-tests.neon
index d6b9aada72..6d3f480c45 100644
--- a/tasks/phpstan/phpstan-baseline-integration-tests.neon
+++ b/tasks/phpstan/phpstan-baseline-integration-tests.neon
@@ -291,7 +291,7 @@ parameters:
 			path: ../../tests/integration/Cron/Workers/SendingQueue/Tasks/PostsTest.php
 
 		-
-			message: "#^Cannot access property \\$post_title on array\\|WP_Post\\|null\\.$#"
+			message: "#^Cannot access property \\$post_title on WP_Post\\|null\\.$#"
 			count: 1
 			path: ../../tests/integration/Cron/Workers/SendingQueue/Tasks/ShortcodesTest.php
 
@@ -595,7 +595,7 @@ parameters:
 			path: ../../tests/integration/Newsletter/Scheduler/PostNotificationTest.php
 
 		-
-			message: "#^Cannot access property \\$post_title on array\\|WP_Post\\|null\\.$#"
+			message: "#^Cannot access property \\$post_title on WP_Post\\|null\\.$#"
 			count: 1
 			path: ../../tests/integration/Newsletter/ShortcodesTest.php
 
diff --git a/tasks/phpstan/phpstan-tests.neon b/tasks/phpstan/phpstan-tests.neon
index df22984957..055cda4a28 100644
--- a/tasks/phpstan/phpstan-tests.neon
+++ b/tasks/phpstan/phpstan-tests.neon
@@ -19,8 +19,6 @@ parameters:
       - PremiumContainerConfigurator.php
       - woocommerce.php
       - function-stubs.php
-  dynamicConstantNames:
-    - WP_DEBUG
   ignoreErrors:
     - '/Parameter #1 \$cssOrXPath of method AcceptanceTester::moveMouseOver\(\) expects string\|null, array<string, string> given./'
     - '/Function expect invoked with 1 parameter, 0 required\./'
@@ -37,9 +35,9 @@ parameters:
   checkMissingTypehints: false
 
 includes:
-  - _phpstan-wp-source.neon # Config file that adds paths to WP source code. Generated from phpstan-wp-source.neon by Robo
   - vendor/phpstan/phpstan-doctrine/extension.neon
   - vendor/phpstan/phpstan-phpunit/extension.neon
+  - vendor/szepeviktor/phpstan-wordpress/extension.neon
   - extensions/CodeceptionExtension/extension.neon
   - phpstan-baseline-unit-tests.neon # https://medium.com/@ondrejmirtes/phpstans-baseline-feature-lets-you-hold-new-code-to-a-higher-standard-e77d815a5dff
   - phpstan-baseline-integration-tests.neon # https://medium.com/@ondrejmirtes/phpstans-baseline-feature-lets-you-hold-new-code-to-a-higher-standard-e77d815a5dff
diff --git a/tasks/phpstan/phpstan.neon b/tasks/phpstan/phpstan.neon
index b0f2d43f42..7265f51202 100644
--- a/tasks/phpstan/phpstan.neon
+++ b/tasks/phpstan/phpstan.neon
@@ -22,7 +22,6 @@ parameters:
       count: 2
   reportUnmatchedIgnoredErrors: true
   dynamicConstantNames:
-    - WP_DEBUG
     - MAILPOET_PREMIUM_INITIALIZED
   doctrine:
     objectManagerLoader: create-entity-manager.php
@@ -37,8 +36,8 @@ parameters:
   excludes_analyse:
     - ../../lib/Config/PopulatorData/Templates
 includes:
-  - _phpstan-wp-source.neon # Config file that adds paths to WP source code. Generated from phpstan-wp-source.neon by Robo
   - vendor/phpstan/phpstan-doctrine/extension.neon
   - vendor/phpstan/phpstan-doctrine/rules.neon
+  - vendor/szepeviktor/phpstan-wordpress/extension.neon
   - phpstan-baseline.neon # https://medium.com/@ondrejmirtes/phpstans-baseline-feature-lets-you-hold-new-code-to-a-higher-standard-e77d815a5dff
   - php-version-dependent-config-libs.php # PHP version detection + configuraton dependent on PHP version