diff --git a/RoboFile.php b/RoboFile.php
index ae97b94e59..df48d9f9a6 100644
--- a/RoboFile.php
+++ b/RoboFile.php
@@ -453,22 +453,32 @@ class RoboFile extends \Robo\Tasks {
   }
 
   function qaPhpstan() {
+    $dir = __DIR__;
+    $task = implode(' ', [
+      'WP_ROOT="' . getenv('WP_ROOT') . '"',
+      'php -d memory_limit=2G',
+      "$dir/tasks/phpstan/vendor/bin/phpstan analyse ",
+      '--level 5',
+    ]);
+
     // PHPStan must be run out of main plugin directory to avoid its autoloading
     // from vendor/autoload.php where some dev dependencies cause conflicts.
-    $dir = __DIR__;
     return $this->collectionBuilder()
       ->taskExec('rm -rf ' . __DIR__ . '/vendor/goaop')
       ->taskExec('rm -rf ' . __DIR__ . '/vendor/nikic')
       ->taskExec('cd ' . __DIR__ . ' && ./tools/vendor/composer.phar dump-autoload')
-      ->taskExec(
-        'WP_ROOT="' . getenv('WP_ROOT') . '" ' .
-        'php -d memory_limit=2G ' .
-        "$dir/tasks/phpstan/vendor/bin/phpstan analyse " .
-        "--configuration $dir/tasks/phpstan/phpstan.neon " .
-        '--level 5 ' .
-        "$dir/lib"
-      )
+
+      // lib
+      ->taskExec($task)
+      ->arg("$dir/lib")
       ->dir(__DIR__ . '/tasks/phpstan')
+
+      // tests
+      ->taskExec($task)
+      ->rawArg('--configuration=phpstan-tests.neon')
+      ->arg("$dir/tests/unit")
+      ->dir(__DIR__ . '/tasks/phpstan')
+
       ->taskExec('cd ' . __DIR__ . ' && ./tools/vendor/composer.phar install')
       ->run();
   }
diff --git a/tasks/phpstan/phpstan-tests.neon b/tasks/phpstan/phpstan-tests.neon
new file mode 100644
index 0000000000..72baacbb3c
--- /dev/null
+++ b/tasks/phpstan/phpstan-tests.neon
@@ -0,0 +1,13 @@
+parameters:
+  mailpoetDir: %rootDir%/../../../../..
+  tmpDir: ../../temp/phpstan
+  bootstrap: bootstrap.php
+  autoload_files:
+    - %mailpoetDir%/vendor/autoload.php
+    - %mailpoetDir%/vendor/codeception/codeception/autoload.php
+    - %mailpoetDir%/vendor/codeception/verify/src/Codeception/function.php
+    - %mailpoetDir%/vendor/codeception/verify/src/Codeception/Verify.php
+  autoload_directories:
+    - %mailpoetDir%/tests/unit
+  dynamicConstantNames:
+    - WP_DEBUG
diff --git a/tests/unit/_bootstrap.php b/tests/unit/_bootstrap.php
index 5e0f3a6519..60a377bab0 100644
--- a/tests/unit/_bootstrap.php
+++ b/tests/unit/_bootstrap.php
@@ -1,7 +1,9 @@
 <?php
 
-function __($text) {
-  return $text;
+if (!function_exists('__')) {
+  function __($text) {
+    return $text;
+  }
 }
 
 // Fix for mocking WPFunctions
@@ -10,8 +12,13 @@ if (!defined('OBJECT')) {
   define( 'OBJECT', 'OBJECT' );
 }
 
-define('ABSPATH', '/');
-define('WP_DEBUG', false);
+if (!defined('ABSPATH')) {
+  define('ABSPATH', '/');
+}
+
+if (!defined('WP_DEBUG')) {
+  define('WP_DEBUG', false);
+}
 
 $console = new \Codeception\Lib\Console\Output([]);