diff --git a/includes/autoload/DrupalAPCClassLoader.php b/includes/autoload/DrupalAPCClassLoader.php
new file mode 100644
index 0000000..fe8f075 100644
--- /dev/null
+++ b/includes/autoload/DrupalAPCClassLoader.php
@@ -0,0 +1,53 @@
+<?php
+
+/**
+ * Class loader proxy implementation that will use APC for caching.
+ */
+class DrupalAPCClassLoader extends DrupalClassLoader {
+
+  /**
+   * Key prefix in APC user cache.
+   *
+   * @var string
+   */
+  protected $prefix;
+
+  /**
+   * Constructor.
+   *
+   * @param string $prefix
+   *   (optional) Prefix to use for APC user cache keys.
+   */
+  public function __construct($prefix = NULL) {
+    $this->prefix = $prefix;
+  }
+
+  /**
+   * Overrides DrupalClassLoader::loadClass().
+   */
+  public function loadClass($classname) {
+
+    if (NULL !== $this->prefix) {
+      $key = $key . $classname;
+    } else {
+      $key = $classname;
+    }
+
+    if ($filename = apc_fetch($key)) {
+      if ((bool)include $filename) {
+        return TRUE;
+      }
+      else {
+        // Key exists but file does not, update the cache.
+        apc_delete($key);
+      }
+    }
+
+    if ($filename = $this->findFile($classname) && (bool)include $filename) {
+      apc_store($key, $filename);
+      return TRUE;
+    }
+
+    return FALSE;
+  }
+}
diff --git a/includes/autoload/DrupalClassLoader.inc b/includes/autoload/DrupalClassLoader.inc
new file mode 100644
index 0000000..70b85fa 100644
--- /dev/null
+++ b/includes/autoload/DrupalClassLoader.inc
@@ -0,0 +1,205 @@
+<?php
+
+/**
+ * @file
+ * Defines the DrupalClassLoader class.
+ */
+
+/**
+ * Drupal core frontal autoloader implementation.
+ *
+ * Credits for PSR-0 support goes to:
+ *  - Jonathan H. Wage <jonwage@gmail.com>
+ *  - Roman S. Borschel <roman@code-factory.org>
+ *  - Matthew Weier O'Phinney <matthew@zend.com>
+ *  - Kris Wallsmith <kris.wallsmith@gmail.com>
+ *  - Fabien Potencier <fabien.potencier@symfony-project.org>
+ *
+ * @see https://gist.github.com/3038217
+ *   For the original code.
+ *
+ * This code is highly similar to the Symfony 2.x ClassLoader component. The
+ * reason we don't use it is to keep PHP 5.2 compatibility.
+ */
+class DrupalClassLoader {
+
+  /**
+   * File extension.
+   *
+   * @var string
+   */
+  protected $fileExtension = '.php';
+
+  /**
+   * Namespace separator.
+   *
+   * @var string
+   */
+  protected $namespaceSeparator = '\\';
+
+  /**
+   * List of registered namespaces.
+   *
+   * @var array
+   *   Keys are namespaces and values are SplClassLoader instances.
+   */
+  protected $namespaces = array();
+
+  /**
+   * Should we use include path or not.
+   *
+   * @var boolean
+   */
+  protected $useIncludePath = TRUE;
+
+  /**
+   * Register single namespace for autoloading.
+   *
+   * @param string $namespace
+   *   Namespace, e.g. MyModule or My\Module
+   * @param string $include_path
+   *   (optional) Root path where to find files, if none given will lookup into
+   *   all include_path root.
+   */
+  public function registerNamespace($namespace, $include_path = NULL) {
+    $this->namespaces[$namespace] = $include_path;
+  }
+
+  /**
+   * Register a set of namespaces at once.
+   *
+   * @param array $namespaces
+   *   Key/value pairs, keys are namespaces and values are include path for
+   *   each namespace.
+   *
+   * @see DrupalClassLoader::registerNamespace()
+   *   For parameters descriptions.
+   */
+  public function registerNamespaces(array $namespaces) {
+    foreach ($namespaces as $namespace => $include_path) {
+      $this->registerNamespace($namespace, $include_path);
+    }
+  }
+
+  /**
+   * Unregister a single namespace.
+   *
+   * The given namespace must be registered using
+   * DrupalClassLoader::unregisterdNamespace().
+   *
+   * @param string $namespace
+   *   Namespace to unregister.
+   */
+  public function unregisterNamespace($namespace) {
+    if (isset($this->namespaces[$namespace])) {
+      $instance = $this->namespaces[$namespace];
+      unset($this->namespaces[$namespace]);
+    }
+  }
+
+  /**
+   * Unregister a set of namespaces at once.
+   *
+   * @param array $namespaces
+   *   Array of which values are namespaces names.
+   */
+  public function unregisterNamespaces(array $namespaces) {
+    foreach ($namespaces as $namespace) {
+      $this->unregister($namespace);
+    }
+  }
+
+  /**
+   * Tells if the current instance handles namespaces.
+   *
+   * @return boolean
+   *   TRUE if at least one namespace has been registered otherwise FALSE.
+   */
+  public function hasNamespaces() {
+    return !empty($this->namespaces);
+  }
+
+  /**
+   * Loads the given class or interface.
+   *
+   * @param string $className
+   *   The name of the class to load.
+   */
+  public function loadClass($classname) {
+    if ($filepath = $this->findFile($classname)) {
+      return (bool)include $filepath;
+    }
+    return false;
+  }
+
+  /**
+   * Resolve a file path.
+   *
+   * This is the heart of PSR-0 implementation. Credits goes to original PSR-0
+   * team, and code is located at https://gist.github.com/3038217
+   *
+   * @param string $className
+   *   The class to load.
+   * @param string $namespace
+   *   The namespace to find it in.
+   * @param string $include_path
+   *   The namespace include path.
+   *
+   * @return string
+   *   File path or NULL if none found.
+   */
+  protected function resolveFileName($classname, $namespace, $include_path = NULL) {
+    if (NULL === $namespace || $namespace === substr($classname, 0, strlen($namespace))) {
+      $filename = '';
+      $namespace = '';
+
+      if (FALSE !== ($lastNsPos = strripos($classname, $this->namespaceSeparator))) {
+        $namespace = substr($classname, 0, $lastNsPos);
+        $classname = substr($classname, $lastNsPos + 1);
+        $filename = str_replace($this->namespaceSeparator, DIRECTORY_SEPARATOR, $namespace) . DIRECTORY_SEPARATOR;
+      }
+
+      $filename .= str_replace('_', DIRECTORY_SEPARATOR, $classname) . $this->fileExtension;
+
+      return ($include_path !== NULL ? $include_path . DIRECTORY_SEPARATOR : '') . $filename;
+    }
+  }
+
+  /**
+   * Find class file.
+   *
+   * @param string $classname
+   *   The name of the class to find.
+   *
+   * @return string
+   *   File path if found otherwise FALSE.
+   */
+  public function findFile($classname) {
+    foreach ($this->namespaces as $namespace => $include_path) {
+      if (($filename = $this->resolveFileName($classname, $namespace, $include_path))) {
+
+        if ($this->useIncludePath) {
+          $filename = stream_resolve_include_path($filename);
+        }
+
+        if (file_exists($filename)) {
+          return $filename;
+        }
+      }
+    }
+  }
+
+  /**
+   * Installs this class loader on the SPL autoload stack.
+   */
+  public function register() {
+    spl_autoload_register(array($this, 'loadClass'));
+  }
+
+  /**
+   * Uninstalls this class loader from the SPL autoloader stack.
+   */
+  public function unregister() {
+    spl_autoload_unregister(array($this, 'loadClass'));
+  }
+}
diff --git a/includes/bootstrap.inc b/includes/bootstrap.inc
index 1fd497d..64fdfff 100644
--- a/includes/bootstrap.inc
+++ b/includes/bootstrap.inc
@@ -2293,6 +2293,13 @@ function _drupal_bootstrap_configuration() {
   timer_start('page');
   // Initialize the configuration, including variables from settings.php.
   drupal_settings_initialize();
+
+  // Initialize the class loaders that don't depend on database.
+  $loader = drupal_autoload_get();
+  if ($namespaces = variable_get('classload_namespaces')) {
+    $loader->registerNamespaces($namespaces);
+  }
+  $loader->register();
 }
 
 /**
@@ -2995,6 +3002,26 @@ function drupal_get_complete_schema($rebuild = FALSE) {
  */
 
 /**
+ * Get core autoloader.
+ *
+ * @return DrupalClassLoader
+ *   The autoloader instance.
+ */
+function drupal_autoload_get() {
+  $loader = &drupal_static(__FUNCTION__);
+  if (!isset($loader)) {
+    require_once DRUPAL_ROOT . '/includes/autoload/DrupalClassLoader.inc';
+    if ((!isset($conf['apc_classloader_enable']) || $conf['apc_classloader_enable']) && extension_loaded('apc')) {
+      require_once DRUPAL_ROOT . '/includes/autoload/DrupalAPCClassLoader.inc';
+      $loader = new DrupalAPCClassLoader();
+    } else {
+      $loader = new DrupalClassLoader();
+    }
+  }
+  return $loader;
+}
+
+/**
  * Confirms that an interface is available.
  *
  * This function is rarely called directly. Instead, it is registered as an
diff --git a/modules/simpletest/simpletest.info b/modules/simpletest/simpletest.info
index 5583c2f..25bdbca 100644
--- a/modules/simpletest/simpletest.info
+++ b/modules/simpletest/simpletest.info
@@ -13,6 +13,7 @@ files[] = tests/ajax.test
 files[] = tests/batch.test
 files[] = tests/bootstrap.test
 files[] = tests/cache.test
+files[] = tests/classloader.test
 files[] = tests/common.test
 files[] = tests/database_test.test
 files[] = tests/entity_crud_hook_test.test
diff --git a/modules/simpletest/tests/classloader.test b/modules/simpletest/tests/classloader.test
new file mode 100644
index 0000000..1d2a9db 100644
--- /dev/null
+++ b/modules/simpletest/tests/classloader.test
@@ -0,0 +1,93 @@
+<?php
+
+/**
+ * @file
+ * Provides unit tests for graph.inc.
+ */
+
+/**
+ * Unit tests for the graph handling features.
+ */
+class ClassLoaderTest extends DrupalUnitTestCase {
+
+  protected $loader;
+
+  protected $apcLoader;
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Class loader',
+      'description' => 'Drupal core PSR-0 compatible autoloader unit tests.',
+      'group' => 'System',
+    );
+  }
+
+  function setUp() {
+    $include_path = dirname(__FILE__) . '/classloader';
+    $namespaces = array(
+      'Baz' => $include_path,
+      'Mixed_Path' => $include_path,
+    );
+
+    if (extension_loaded('apc')) {
+      $this->apcLoader = new DrupalAPCClassLoader();
+      $this->apcLoader->registerNamespaces($namespaces);
+    }
+
+    $this->loader = new DrupalClassLoader();
+    $this->loader->registerNamespaces($namespaces);
+
+    parent::setUp();
+  }
+
+  function tearDown() {
+    unset($this->loader);
+
+    parent::tearDown();
+  }
+
+  /**
+   * Test autoloader register feature.
+   */
+  function testRegister() {
+    // Ensure no legacy autoloader.
+    spl_autoload_unregister('drupal_autoload_class');
+    spl_autoload_unregister('drupal_autoload_interface');
+
+    $this->loader->register();
+    $this->assertTrue(class_exists('Baz_ClassExists'));
+    $this->loader->unregister();
+
+    // Restore legacy autoloader.
+    spl_autoload_register('drupal_autoload_class');
+    spl_autoload_register('drupal_autoload_interface');
+  }
+
+  /**
+   * PHP 5.2 / PEAR coding style tests.
+   */
+  function testLoadFile() {
+    $this->assertNotNull($this->loader->findFile('Baz_Bar'));
+  }
+
+  /**
+   * PHP 5.3 / Namespaces coding style tests.
+   */
+  function testNamespaces() {
+    // @todo Ensure a check to avoid testing this if PHP 5.2.
+    $this->assertNotNull($this->loader->findFile('Baz\SomeClass'));
+    $this->assertNotNull($this->loader->findFile('Mixed_Path\Other'));
+    $this->assertNotNull($this->loader->findFile('Baz\Some_OtherClass'));
+  }
+
+  /**
+   * PHP 5.2 / PEAR coding style tests with APC loader.
+   */
+  function testAPCLoadFile() {
+    if (!extension_loaded('apc')) {
+      return;
+    }
+    $this->assertNotNull($this->apcLoader->findFile('Baz_Bar'));
+  }
+}
+
diff --git a/modules/simpletest/tests/classloader/Baz/Bar.php b/modules/simpletest/tests/classloader/Baz/Bar.php
new file mode 100644
index 0000000..c72f334 100644
--- /dev/null
+++ b/modules/simpletest/tests/classloader/Baz/Bar.php
@@ -0,0 +1,7 @@
+<?php
+
+/**
+ * PEAR coding convention.
+ */
+class Baz_Bar {  
+}
diff --git a/modules/simpletest/tests/classloader/Baz/ClassExists.php b/modules/simpletest/tests/classloader/Baz/ClassExists.php
new file mode 100644
index 0000000..6f11230 100644
--- /dev/null
+++ b/modules/simpletest/tests/classloader/Baz/ClassExists.php
@@ -0,0 +1,3 @@
+<?php
+
+class Baz_ClassExists {}
diff --git a/modules/simpletest/tests/classloader/Baz/Some/OtherClass.php b/modules/simpletest/tests/classloader/Baz/Some/OtherClass.php
new file mode 100644
index 0000000..64005e3 100644
--- /dev/null
+++ b/modules/simpletest/tests/classloader/Baz/Some/OtherClass.php
@@ -0,0 +1,8 @@
+<?php
+
+namespace Baz;
+
+/**
+ * PSR-0 PHP 5.3 class with _ in class name.
+ */
+class Some_OtherClass {}
diff --git a/modules/simpletest/tests/classloader/Baz/SomeClass.php b/modules/simpletest/tests/classloader/Baz/SomeClass.php
new file mode 100644
index 0000000..85f9df2 100644
--- /dev/null
+++ b/modules/simpletest/tests/classloader/Baz/SomeClass.php
@@ -0,0 +1,8 @@
+<?php
+
+namespace Baz;
+
+/**
+ * PSR-0 PHP 5.3 class.
+ */
+class SomeClass {}
diff --git a/modules/simpletest/tests/classloader/Mixed_Path/Other.php b/modules/simpletest/tests/classloader/Mixed_Path/Other.php
new file mode 100644
index 0000000..f80600b 100644
--- /dev/null
+++ b/modules/simpletest/tests/classloader/Mixed_Path/Other.php
@@ -0,0 +1,9 @@
+<?php
+
+namespace Mixed_Path;
+
+/**
+ * PHP 5.3 namespace code with _ inside the namespace.
+ */
+class Other {
+}