diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -3545,8 +3545,8 @@
 /**
  * Instantiates and statically caches the correct class for a PHP loader.
  *
- * By default, this returns an instance of the Drupal\Component\PhpLoader\MTimeProtectedLoader
- * class.
+ * By default, this returns an instance of the
+ * Drupal\Component\PhpLoader\MTimeProtectedLoader class.
  *
  * Classes implementing Drupal\Component\PhpLoader\PhpLoaderInterface can
  * register themselves both as a default implementation and for specific bins.
diff -u b/core/lib/Drupal/Component/PhpLoader/MTimeProtectedLoader.php b/core/lib/Drupal/Component/PhpLoader/MTimeProtectedLoader.php
--- b/core/lib/Drupal/Component/PhpLoader/MTimeProtectedLoader.php
+++ b/core/lib/Drupal/Component/PhpLoader/MTimeProtectedLoader.php
@@ -1,8 +1,13 @@
 <?php
 
+/**
+ * @file
+ * Definition of Drupal\Component\PhpLoader\MTimeProtectedLoader.
+ */
 namespace Drupal\Component\PhpLoader;
 
 class MTimeProtectedLoader implements PhpLoaderInterface {
+
   /**
    * The path to the directory containing the files.
    *
@@ -54,7 +59,7 @@
       mkdir($this->prefix, 0700, TRUE);
       chmod($this->prefix, 0700);
     }
-    if (!@file_put_contents($original_path, $data)) {
+    if (!$length = @file_put_contents($original_path, $data)) {
       return FALSE;
     }
     chmod($original_path, 0400);
@@ -96,6 +101,7 @@
       chmod($dir, 0100);
       $loop++;
     }
+    return $length;
   }
 
   /**
@@ -109,6 +115,9 @@
 
   /**
    * Removes everything in a directory, leaving it empty.
+   *
+   * @param $dir
+   *   The dir to be emptied out.
    */
   protected function cleanDir($dir) {
     chmod($dir, 0700);
@@ -120,9 +129,16 @@
   /**
    * Constructs the secret path based on the filename and the mtime.
    *
+   * This function returns a full path starting with $dir and ending in .php,
+   * the filename itself will be a secure hash based on $filename, the secret
+   * based to the constructor and $dir_mtime.
+   *
    * @param string $dir
+   *   The directory containing the file.
    * @param string $filename
+   *   The filename.
    * @param int $dir_mtime
+   *   The mtime of $dir.
    */
   protected function getPath($dir, $filename, $dir_mtime) {
     return $dir . '/' . hash_hmac('sha256', $filename, $this->secret . $dir_mtime) . '.php';
diff -u b/core/lib/Drupal/Component/PhpLoader/NativeLoader.php b/core/lib/Drupal/Component/PhpLoader/NativeLoader.php
--- b/core/lib/Drupal/Component/PhpLoader/NativeLoader.php
+++ b/core/lib/Drupal/Component/PhpLoader/NativeLoader.php
@@ -1,7 +1,7 @@
 <?php
 /**
  * @file
- * Definition of Drupal\Component\PhpLoader\NativeLoader
+ * Definition of Drupal\Component\PhpLoader\NativeLoader.
  */
 
 namespace Drupal\Component\PhpLoader;
@@ -10,6 +10,9 @@
  * The simplest PHP loader: just uses the native methods.
  */
 class NativeLoader implements PhpLoaderInterface {
+  /**
+   * The path prefix where the files should be stored.
+   */
   protected $prefix;
 
 
@@ -36,7 +39,9 @@
    * Implements Drupal\Component\PhpLoader\PhpLoaderInterface::write()
    */
   public function write($filename, $data) {
-    return file_put_contents($this->prefix . '/' . $filename, $data);
+    $path = $this->prefix . '/' . $filename;
+    @mkdir(dirname($path), 0700, TRUE);
+    return file_put_contents($path, $data);
   }
 
   /**
diff -u b/core/lib/Drupal/Component/PhpLoader/PhpLoaderInterface.php b/core/lib/Drupal/Component/PhpLoader/PhpLoaderInterface.php
--- b/core/lib/Drupal/Component/PhpLoader/PhpLoaderInterface.php
+++ b/core/lib/Drupal/Component/PhpLoader/PhpLoaderInterface.php
@@ -1,10 +1,16 @@
 <?php
 
+/**
+ * @file
+ * Definition of Drupal\system\Tests\PhpLoader\PhpLoaderInterface.
+ */
+
 namespace Drupal\Component\PhpLoader;
 
 interface PhpLoaderInterface {
+
   /**
-   * Include a PHP file.
+   * Includes a PHP file.
    *
    * @param string $filename
    *   The filename. Can be a relative path.
@@ -15,7 +21,7 @@
   public function includePhp($filename);
 
   /**
-   * Write a PHP file.
+   * Writes a PHP file.
    *
    * @param string $filename
    *   The filename. Can be a relative path.
@@ -23,12 +29,13 @@
    *    The PHP code to be written.
    *
    * @return
-   *   FALSE if the write failed.
+   *   FALSE if the write failed, anything but FALSE on success. For example,
+   *   core implementations return the number of bytes written.
    */
   public function write($filename, $data);
 
   /**
-   * Delete a PHP file.
+   * Deletes a PHP file.
    *
    * @param string $filename
    *   The filename to be deleted. Can be a relative path.
diff -u b/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
--- b/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -522,7 +522,7 @@
 }
 
 /**
- * Recursively delete a directory regardless of permissions (if possible).
+ * Deletes a directory recursively regardless of permissions (if possible).
  *
  * @param $path
  *   The path that will be obliterated.
diff -u b/core/modules/system/lib/Drupal/system/Tests/PhpLoader/MTimeProtectedLoaderTest.php b/core/modules/system/lib/Drupal/system/Tests/PhpLoader/MTimeProtectedLoaderTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/PhpLoader/MTimeProtectedLoaderTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/PhpLoader/MTimeProtectedLoaderTest.php
@@ -9,6 +9,9 @@
 
 use Drupal\simpletest\UnitTestBase;
 
+/**
+ * Tests the directory mtime based PHP loader implementation.
+ */
 class MTimeProtectedLoaderTest extends UnitTestBase {
 
   public static function getInfo() {
@@ -19,6 +22,13 @@
     );
   }
 
+  /**
+   * Tests the directory mtime based PHP loader implementation.
+   *
+   * This test first writes a file containing the definition of a non-existing
+   * function returning a random. Then loads it back, calls the function and
+   * asserts the return is correct.
+   */
   public function testMTimeProtectedLoader() {
     $conf['loader_classes'] = array('default' =>
       array(