fixes sebastianbergmann#3881

src/Runner/StandardTestSuiteLoader.php

@@ -52,6 +52,10 @@ public function load(string $suiteClassName, string $suiteClassFile = ''): Refle
                 );
             }
 
+            if ($class->isAbstract()) {
+                continue;
+            }
+
             if (\substr($loadedClass, $offset) === $suiteClassName &&
                 $class->getFileName() == $filename) {
                 $suiteClassName = $loadedClass;

tests/end-to-end/regression/GitHub/3881.phpt

@@ -0,0 +1,18 @@
+--TEST--
+https://github.com/sebastianbergmann/phpunit/issues/3881
+--FILE--
+<?php declare(strict_types=1);
+$_SERVER['argv'][1] = '--no-configuration';
+$_SERVER['argv'][3] = __DIR__ . '/3881/Issue3881Test.php';
+
+require __DIR__ . '/../../../bootstrap.php';
+
+PHPUnit\TextUI\Command::main();
+--EXPECTF--
+PHPUnit %s by Sebastian Bergmann and contributors.
+
+.                                                                   1 / 1 (100%)
+
+Time: %s, Memory: %s
+
+OK (1 test, 1 assertion)

tests/end-to-end/regression/GitHub/3881/Issue3881Test.php

@@ -0,0 +1,14 @@
+<?php
+use PHPUnit\Framework\TestCase;
+
+abstract class AbstractIssue3881Test extends TestCase
+{
+    public function testOne(): void
+    {
+        $this->assertTrue(true);
+    }
+}
+
+final class Issue3881Test extends AbstractIssue3881Test
+{
+}