diff --git a/lib/classes/plugin_manager.php b/lib/classes/plugin_manager.php
index af27537f84e..254e04f2281 100644
--- a/lib/classes/plugin_manager.php
+++ b/lib/classes/plugin_manager.php
@@ -86,10 +86,10 @@ class core_plugin_manager {
      * @return core_plugin_manager the singleton instance
      */
     public static function instance() {
-        if (is_null(self::$singletoninstance)) {
-            self::$singletoninstance = new self();
+        if (is_null(static::$singletoninstance)) {
+            static::$singletoninstance = new static();
         }
-        return self::$singletoninstance;
+        return static::$singletoninstance;
     }
 
     /**
@@ -98,15 +98,15 @@ class core_plugin_manager {
      */
     public static function reset_caches($phpunitreset = false) {
         if ($phpunitreset) {
-            self::$singletoninstance = null;
+            static::$singletoninstance = null;
         } else {
-            if (self::$singletoninstance) {
-                self::$singletoninstance->pluginsinfo = null;
-                self::$singletoninstance->subpluginsinfo = null;
-                self::$singletoninstance->installedplugins = null;
-                self::$singletoninstance->enabledplugins = null;
-                self::$singletoninstance->presentplugins = null;
-                self::$singletoninstance->plugintypes = null;
+            if (static::$singletoninstance) {
+                static::$singletoninstance->pluginsinfo = null;
+                static::$singletoninstance->subpluginsinfo = null;
+                static::$singletoninstance->installedplugins = null;
+                static::$singletoninstance->enabledplugins = null;
+                static::$singletoninstance->presentplugins = null;
+                static::$singletoninstance->plugintypes = null;
             }
         }
         $cache = cache::make('core', 'plugin_manager');
@@ -238,7 +238,7 @@ class core_plugin_manager {
 
         $plugintypes = core_component::get_plugin_types();
         foreach ($plugintypes as $plugintype => $fulldir) {
-            $plugininfoclass = self::resolve_plugininfo_class($plugintype);
+            $plugininfoclass = static::resolve_plugininfo_class($plugintype);
             if (class_exists($plugininfoclass)) {
                 $enabled = $plugininfoclass::get_enabled_plugins();
                 if (!is_array($enabled)) {
@@ -374,13 +374,13 @@ class core_plugin_manager {
 
         if (!isset($types[$type])) {
             // Orphaned subplugins!
-            $plugintypeclass = self::resolve_plugininfo_class($type);
+            $plugintypeclass = static::resolve_plugininfo_class($type);
             $this->pluginsinfo[$type] = $plugintypeclass::get_plugins($type, null, $plugintypeclass);
             return $this->pluginsinfo[$type];
         }
 
         /** @var \core\plugininfo\base $plugintypeclass */
-        $plugintypeclass = self::resolve_plugininfo_class($type);
+        $plugintypeclass = static::resolve_plugininfo_class($type);
         $plugins = $plugintypeclass::get_plugins($type, $types[$type], $plugintypeclass);
         $this->pluginsinfo[$type] = $plugins;
 
@@ -1297,7 +1297,7 @@ class core_plugin_manager {
             return false;
         }
 
-        if ($pluginfo->get_status() === self::PLUGIN_STATUS_NEW) {
+        if ($pluginfo->get_status() === static::PLUGIN_STATUS_NEW) {
             // The plugin is not installed. It should be either installed or removed from the disk.
             // Relying on this temporary state may be tricky.
             return false;
diff --git a/lib/tests/fixtures/testable_plugin_manager.php b/lib/tests/fixtures/testable_plugin_manager.php
new file mode 100644
index 00000000000..847bff43ab9
--- /dev/null
+++ b/lib/tests/fixtures/testable_plugin_manager.php
@@ -0,0 +1,38 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Provides testable_core_plugin_manager class.
+ *
+ * @package     core
+ * @category    test
+ * @copyright   2015 David Mudrak <david@moodle.com>
+ * @license     http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+defined('MOODLE_INTERNAL') || die();
+
+/**
+ * Testable variant of the core_plugin_manager
+ *
+ * @copyright 2015 David Mudrak <david@moodle.com>
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class testable_core_plugin_manager extends core_plugin_manager {
+
+    /** @var testable_core_plugin_manager holds the singleton instance */
+    protected static $singletoninstance;
+}
diff --git a/lib/tests/plugin_manager_test.php b/lib/tests/plugin_manager_test.php
index d3ff088c1e6..f9726e878cb 100644
--- a/lib/tests/plugin_manager_test.php
+++ b/lib/tests/plugin_manager_test.php
@@ -25,21 +25,31 @@
 
 defined('MOODLE_INTERNAL') || die();
 
+global $CFG;
+require_once($CFG->dirroot.'/lib/tests/fixtures/testable_plugin_manager.php');
+
 /**
  * Tests of the basic API of the plugin manager.
  */
 class core_plugin_manager_testcase extends advanced_testcase {
 
     public function test_instance() {
-        $pluginman = core_plugin_manager::instance();
-        $this->assertInstanceOf('core_plugin_manager', $pluginman);
+        $pluginman1 = core_plugin_manager::instance();
+        $this->assertInstanceOf('core_plugin_manager', $pluginman1);
         $pluginman2 = core_plugin_manager::instance();
-        $this->assertSame($pluginman, $pluginman2);
+        $this->assertSame($pluginman1, $pluginman2);
+        $pluginman3 = testable_core_plugin_manager::instance();
+        $this->assertInstanceOf('core_plugin_manager', $pluginman3);
+        $this->assertInstanceOf('testable_core_plugin_manager', $pluginman3);
+        $pluginman4 = testable_core_plugin_manager::instance();
+        $this->assertSame($pluginman3, $pluginman4);
+        $this->assertNotSame($pluginman1, $pluginman3);
     }
 
     public function test_reset_caches() {
         // Make sure there are no warnings or errors.
         core_plugin_manager::reset_caches();
+        testable_core_plugin_manager::reset_caches();
     }
 
     public function test_get_plugin_types() {
@@ -95,12 +105,23 @@ class core_plugin_manager_testcase extends advanced_testcase {
     }
 
     public function test_get_plugins() {
-        $plugininfos = core_plugin_manager::instance()->get_plugins();
-        foreach ($plugininfos as $type => $infos) {
+        $plugininfos1 = core_plugin_manager::instance()->get_plugins();
+        foreach ($plugininfos1 as $type => $infos) {
             foreach ($infos as $name => $info) {
                 $this->assertInstanceOf('\core\plugininfo\base', $info);
             }
         }
+
+        // The testable variant of the manager holds its own tree of the
+        // plugininfo objects.
+        $plugininfos2 = testable_core_plugin_manager::instance()->get_plugins();
+        $this->assertNotSame($plugininfos1['mod']['forum'], $plugininfos2['mod']['forum']);
+
+        // Singletons of each manager class share the same tree.
+        $plugininfos3 = core_plugin_manager::instance()->get_plugins();
+        $this->assertSame($plugininfos1['mod']['forum'], $plugininfos3['mod']['forum']);
+        $plugininfos4 = testable_core_plugin_manager::instance()->get_plugins();
+        $this->assertSame($plugininfos2['mod']['forum'], $plugininfos4['mod']['forum']);
     }
 
     public function test_get_plugins_of_type() {