Doc: Add Testing Singletons paragraph

refs #4548

doc/test/php_tests.md

@@ -28,6 +28,37 @@ modules/mymodule/library/MyModule/Helper/MyClass.php.
         }
     }
 
+## Testing Singletons
+
+When test methods **modify static** class properties (which is the case when using singletons), add the PHPUnit
+[`@backupStaticAttributes enabled`](http://phpunit.de/manual/3.7/en/appendixes.annotations.html#appendixes.annotations.backupStaticAttributes)
+annotation to their [DockBlock](http://www.phpdoc.org/docs/latest/for-users/phpdoc/basic-syntax.html#what-is-a-docblock)
+in order to backup and restore static attributes before and after the method execution respectively. For reference you
+should **document** that the test interacts with static attributes:
+
+    <?php
+
+    namespace My\Test;
+
+    use \PHPUnit_Framework_TestCase;
+    use My\CheesecakeFactory;
+
+    class SingletonTest extends PHPUnit_Framework_TestCase
+    {
+        /**
+         * Interact with static attributes
+         *
+         * Utilizes singleton CheesecakeFactory
+         *
+         * @backupStaticAttributes enabled
+         */
+        public function testThatInteractsWithStaticAttributes()
+        {
+            CheesecakeFactory::setOpeningHours(24);
+            // ...
+        }
+    }
+
 ## Requirements and the dependency mess
 
 ### spl_autoload_register vs. require
@@ -103,4 +134,4 @@ just use the requireLibrary method:
 
 If you compare the first approach with the last one you will notice that, even if we produced more code in the end, our
 test is more verbose in what it is doing. When someone is updating your test, he should easily see what tests are existing
-and what scenarios are missing.
+and what scenarios are missing.