diff --git a/tests/test_framework/phpbb_functional_test_case.php b/tests/test_framework/phpbb_functional_test_case.php
index 1b47cbe125..16ed9f5a1e 100644
--- a/tests/test_framework/phpbb_functional_test_case.php
+++ b/tests/test_framework/phpbb_functional_test_case.php
@@ -257,8 +257,30 @@ class phpbb_functional_test_case extends phpbb_test_case
 		$form = $crawler->selectButton('submit')->form();
 
 		$crawler = self::submit($form);
+		$config_writable = strpos($crawler->filter('#main')->text(), 'It was not possible to write the configuration file.') === false;
+		$config_php_data = phpbb_create_config_file_data(self::$config, self::$config['dbms'], array(), true, true);
+
+		if (!$config_writable)
+		{
+			// phpBB could not write to the config.php file, so we have to "Download" it.
+			self::assertContains('Download config', $crawler->filter('#main')->text());
+
+			file_put_contents($config_file, $config_php_data);
+
+			$form = $crawler->selectButton('dldone')->form();
+			$crawler = self::submit($form);
+		}
+
 		self::assertContains('The configuration file has been written.', $crawler->filter('#main')->text());
-		file_put_contents($config_file, phpbb_create_config_file_data(self::$config, self::$config['dbms'], array(), true, true));
+
+		// Overwrite the config.php file generated by phpBB in order to get the
+		// DEBUG constants defined if possible. It should be possible when unit
+		// tests run as the same user as phpBB.
+		if ($config_writable && is_writable($config_file))
+		{
+			file_put_contents($config_file, $config_php_data);
+		}
+
 		$form = $crawler->selectButton('submit')->form();
 
 		$crawler = self::submit($form);