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
@@ -868,16 +868,18 @@
   /**
    * Creates simpletest_settings.php file.
    *
-   * @param array $lines
-   *   Lines to add to the simpletest_settings.php, for eaxmple:
+   * @param array $conf
+   *   Conf overrides to add to the simpletest_settings.php, for eaxmple:
    *   @code
    *     array(
-   *       "\$conf['page_cache_without_database'] = TRUE;",
+   *       'page_cache_without_database' = TRUE,
    *     );
    *   @endcode
    */
-  protected function createSimpletestSettingsFile($lines = array()) {
-    $header = <<<EOF
+  protected function createSimpletestSettingsFile($conf = array()) {
+    include_once './core/includes/utility.inc';
+
+    $contents = <<<EOF
 <?php
 /**
  * @file
@@ -889,6 +891,9 @@
 
-    $contents = $header . implode("\n", $lines) . "\n";
+    $contents .= "\$conf += " . drupal_var_export($conf) . ";\n";
     file_put_contents($this->additionalSettingsDirectory . '/simpletest_settings.php', $contents);
   }
 
+  protected function deleteSimpletestSettingsFile() {
+    unlink($this->additionalSettingsDirectory . '/simpletest_settings.php');
+  }
 }
diff -u b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
@@ -115,15 +115,22 @@
    * Test ability for settings.php to specify a non-database backend for the page cache.
    */
   function testPageCacheWithoutDatabase() {
-    $conf_lines = array(
-      "\$conf['page_cache_without_database'] = TRUE;",
-      "\$conf['cache_backends'][] = 'core/modules/system/tests/modules/bootstrap_test/lib/Drupal/bootstrap_test/Cache/TestBackend.php';",
-      "\$conf['cache_classes']['cache'] = 'Drupal\\Core\\Cache\\DatabaseBackend';",
-      "\$conf['cache_classes']['page'] = 'Drupal\\bootstrap_test\\Cache\\TestBackend';",
+    $conf = array(
+      'page_cache_without_database' => TRUE,
+      'cache_backends' => array(
+        'core/modules/system/tests/modules/bootstrap_test/lib/Drupal/bootstrap_test/Cache/TestBackend.php',
+      ),
+      'cache_classes' => array(
+        'cache' => 'Drupal\Core\Cache\DatabaseBackend',
+        'page' => 'Drupal\bootstrap_test\Cache\TestBackend',
+      ),
     );
-    $this->createSimpletestSettingsFile($conf_lines);
+
+    $this->createSimpletestSettingsFile($conf);
     $this->drupalGet('');
     $this->assertText('Page returned by Drupal\bootstrap_test\Cache\TestBackend');
+    // Have to clean up settings file so that is does not affect other tests
+    $this->deleteSimpletestSettingsFile();
   }
 
   /**