diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 4f5e692..13f9dd6 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -304,9 +304,17 @@ function simpletest_phpunit_configuration_filepath() {
  *   The results as returned by exec().
  */
 function simpletest_phpunit_run_command(array $unescaped_test_classnames, $phpunit_file, &$status = NULL) {
+  global $base_url;
   // Setup an environment variable containing the database connection so that
   // functional tests can connect to the database.
   putenv('SIMPLETEST_DB=' . Database::getConnectionInfoAsUrl());
+
+  // Setup an environment variable containing the base URL so that functional
+  // tests can browser the site under test.
+  $existing_simpletest_base_url = getenv('SIMPLETEST_BASE_URL');
+  if ($base_url) {
+    putenv('SIMPLETEST_BASE_URL=' . $base_url);
+  }
   $phpunit_bin = simpletest_phpunit_command();
 
   $command = array(
@@ -344,6 +352,9 @@ function simpletest_phpunit_run_command(array $unescaped_test_classnames, $phpun
 
   chdir($old_cwd);
   putenv('SIMPLETEST_DB=');
+  if ($base_url) {
+    putenv('SIMPLETEST_BASE_URL=' . $existing_simpletest_base_url);
+  }
   return $ret;
 }
 
diff --git a/core/modules/simpletest/src/Form/SimpletestTestForm.php b/core/modules/simpletest/src/Form/SimpletestTestForm.php
index e645705..8ad5fe2 100644
--- a/core/modules/simpletest/src/Form/SimpletestTestForm.php
+++ b/core/modules/simpletest/src/Form/SimpletestTestForm.php
@@ -199,7 +199,6 @@ public function buildForm(array $form, FormStateInterface $form_state) {
    * {@inheritdoc}
    */
   public function submitForm(array &$form, FormStateInterface $form_state) {
-    global $base_url;
     // Test discovery does not run upon form submission.
     simpletest_classloader_register();
 
@@ -219,7 +218,6 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
 
     $tests_list = array_filter($form_state->getValue('tests'));
     if (!empty($tests_list)) {
-      putenv('SIMPLETEST_BASE_URL=' . $base_url);
       $test_id = simpletest_run_tests($tests_list, 'drupal');
       $form_state->setRedirect(
         'simpletest.result_form',