diff -u b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
--- b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -889,7 +889,7 @@
 
 EOF;
 
-    $contents .= "\$conf += " . drupal_var_export($conf) . ";\n";
+    $contents .= "\$conf = " . drupal_var_export($conf) . " + \$conf;\n";
     file_put_contents($this->additionalSettingsDirectory . '/simpletest_settings.php', $contents);
   }
 }