Detect premature end of PHPUnit's main PHP process

src/Framework/TestCase.php

@@ -352,7 +352,12 @@ final public function run(): void
         }
 
         if (!$this->shouldRunInSeparateProcess() || $this->requirementsNotSatisfied()) {
-            (new TestRunner)->run($this);
+            try {
+                ShutdownHandler::setMessage(sprintf('Fatal error: Premature end of PHP process when running %s.', $this->toString()));
+                (new TestRunner)->run($this);
+            } finally {
+                ShutdownHandler::resetMessage();
+            }
 
             return;
         }

src/Runner/ShutdownHandler.php

@@ -39,6 +39,7 @@ private static function register(): void
             return;
         }
 
+        self::$registered = true;
         register_shutdown_function(static function (): void
         {
             print self::$message;

tests/end-to-end/regression/6234.phpt

@@ -0,0 +1,17 @@
+--TEST--
+https://github.com/sebastianbergmann/phpunit/issues/6234
+--FILE--
+<?php declare(strict_types=1);
+$_SERVER['argv'][] = '--do-not-cache-result';
+$_SERVER['argv'][] = '--no-configuration';
+$_SERVER['argv'][] = __DIR__ . '/6234/Issue6234Test.php';
+
+require_once __DIR__ . '/../../bootstrap.php';
+
+(new PHPUnit\TextUI\Application)->run($_SERVER['argv']);
+--EXPECTF--
+PHPUnit %s by Sebastian Bergmann and contributors.
+
+Runtime: %s
+
+Fatal error: Premature end of PHP process when running PHPUnit\TestFixture\Issue6234Test::testExitWithoutProcessIsolation.

tests/end-to-end/regression/6234/Issue6234Test.php

@@ -0,0 +1,23 @@
+<?php declare(strict_types=1);
+/*
+ * This file is part of PHPUnit.
+ *
+ * (c) Sebastian Bergmann <[email protected]>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+namespace PHPUnit\TestFixture;
+
+use PHPUnit\Framework\TestCase;
+
+final class Issue6234Test extends TestCase
+{
+    public function testExitWithoutProcessIsolation(): void
+    {
+        $this->assertTrue(true);
+        $this->assertTrue(true);
+
+        exit(1);
+    }
+}