diff --git a/core/phpunit.xml.dist b/core/phpunit.xml.dist
index 951bd55..a9bef6d 100644
--- a/core/phpunit.xml.dist
+++ b/core/phpunit.xml.dist
@@ -6,8 +6,11 @@
  result printer that links to the html output results for functional tests.
  Unfortunately, this breaks the output of PHPStorm's PHPUnit runner. However, if
  using the command line you can add
- - -printer="\Drupal\Tests\Listeners\HtmlOutputPrinter" to use it (note there
- should be no spaces between the hyphens).
+ - -printerClass="\Drupal\Tests\Listeners\HtmlOutputPrinter" to use it (note
+ there should be no spaces between the hyphens).
+-->
+<!-- PHPUnit expects functional tests to be run with either a privileged user
+ or your Unix user. See core/tests/README.md for details.
 -->
 <phpunit bootstrap="tests/bootstrap.php" colors="true"
          beStrictAboutTestsThatDoNotTestAnything="true"
diff --git a/core/tests/README.md b/core/tests/README.md
index dfd1154..4661173 100644
--- a/core/tests/README.md
+++ b/core/tests/README.md
@@ -13,3 +13,17 @@
   ./vendor/bin/phpunit -c core --testsuite functional
   ./vendor/bin/phpunit -c core --testsuite functional-javascript
   ```
+
+Note: functional tests have to be invoked with a user in the same group as the
+web server user. You can either configure Apache (or nginx) to run as your own
+Unix user or run tests as a privileged user instead.
+
+E.g. if you're running Apache and the default user is `www-data`, the above
+functional tests will thus have to be invoked this way:
+
+```
+export SIMPLETEST_DB='mysql://root@localhost/dev_d8'
+export SIMPLETEST_BASE_URL='http://d8.dev'
+sudo -u www-data ./vendor/bin/phpunit -c core --testsuite functional
+sudo -u www-data ./vendor/bin/phpunit -c core --testsuite functional-javascript
+```