fix: Setting error handler inside an isolated test may cause test failure on PHP 8.3

src/Util/PHP/Template/TestCaseClass.tpl

@@ -66,6 +66,8 @@ function __phpunit_run_isolated_test()
 
     ini_set('xdebug.scream', '0');
 
+    set_error_handler('__phpunit_error_handler');
+
     // Not every STDOUT target stream is rewindable
     @rewind(STDOUT);
 
@@ -79,6 +81,8 @@ function __phpunit_run_isolated_test()
         }
     }
 
+    restore_error_handler();
+
     file_put_contents(
         '{processResultFile}',
         serialize(

src/Util/PHP/Template/TestCaseMethod.tpl

@@ -31,6 +31,11 @@ if ($composerAutoload) {
     require $phar;
 }
 
+function __phpunit_error_handler($errno, $errstr, $errfile, $errline)
+{
+   return true;
+}
+
 function __phpunit_run_isolated_test()
 {
     $dispatcher = Facade::instance()->initForIsolation(
@@ -66,6 +71,8 @@ function __phpunit_run_isolated_test()
 
     ini_set('xdebug.scream', '0');
 
+    set_error_handler('__phpunit_error_handler');
+
     // Not every STDOUT target stream is rewindable
     @rewind(STDOUT);
 
@@ -79,6 +86,8 @@ function __phpunit_run_isolated_test()
         }
     }
 
+    restore_error_handler();
+
     file_put_contents(
         '{processResultFile}',
         serialize(
@@ -94,11 +103,6 @@ function __phpunit_run_isolated_test()
     );
 }
 
-function __phpunit_error_handler($errno, $errstr, $errfile, $errline)
-{
-   return true;
-}
-
 set_error_handler('__phpunit_error_handler');
 
 {constants}

tests/end-to-end/regression/5595.phpt

@@ -0,0 +1,21 @@
+--TEST--
+https://github.com/sebastianbergmann/phpunit/issues/5595
+--FILE--
+<?php declare(strict_types=1);
+$_SERVER['argv'][] = '--do-not-cache-result';
+$_SERVER['argv'][] = '--no-configuration';
+$_SERVER['argv'][]  = '--process-isolation';
+$_SERVER['argv'][]  = __DIR__ . '/5595/Issue5595Test.php';
+
+require_once __DIR__ . '/../../bootstrap.php';
+(new PHPUnit\TextUI\Application)->run($_SERVER['argv']);
+--EXPECTF--
+PHPUnit %s by Sebastian Bergmann and contributors.
+
+Runtime: %s
+
+.                                                                   1 / 1 (100%)
+
+Time: %s, Memory: %s
+
+OK (1 test, 1 assertion)

tests/end-to-end/regression/5595/Issue5595Test.php

@@ -0,0 +1,31 @@
+<?php declare(strict_types=1);
+/*
+ * This file is part of PHPUnit.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+namespace PHPUnit\TestFixture;
+
+use function set_error_handler;
+use PHPUnit\Framework\Attributes\RunInSeparateProcess;
+use PHPUnit\Framework\TestCase;
+
+class Issue5595Test extends TestCase
+{
+    protected function setUp(): void
+    {
+        set_error_handler(static function (): bool
+        {
+            return true;
+        });
+    }
+
+    #[RunInSeparateProcess]
+    public function test(): void
+    {
+        $this->assertTrue(true);
+    }
+}