minor symfony#11258 Added the docs for the VarExporter component (javiereguiluz)

This PR was merged into the 4.2 branch.

Discussion
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Added the docs for the VarExporter component

Fixes symfony#10230.

Commits
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a67bb16 Added the docs for the VarExporter component

Author: javiereguiluz

components/var_exporter.rst

@@ -0,0 +1,134 @@
+.. index::
+   single: VarExporter
+   single: Components; VarExporter
+
+The VarExporter Component
+=========================
+
+    The VarExporter component exports any serializable PHP data structure to
+    plain PHP code and allows to instantiate and populate objects without
+    calling their constructors.
+
+Installation
+------------
+
+.. code-block:: terminal
+
+    $ composer require --dev symfony/var-exporter
+
+Alternatively, you can clone the `<https://github.com/symfony/var-exporter>`_ repository.
+
+.. include:: /components/require_autoload.rst.inc
+
+Exporting/Serializing Variables
+-------------------------------
+
+The main feature of this component is to serialize PHP data structures to plain
+PHP code, similar to PHP's :phpfunction:`var_export` function::
+
+    use Symfony\Component\VarExporter\VarExporter;
+
+    $exported = VarExporter::export($someVariable);
+    // store the $exported data in some file or cache system for later reuse
+    $data = file_put_contents('exported.php', $exported);
+
+    // later, regenerate the original variable when you need it
+    $regeneratedVariable = require 'exported.php';
+
+The reason to use this component instead of ``serialize()`` or ``igbinary`` is
+performance: thanks to `OPcache`_, the resulting code is significantly faster
+and more memory efficient than using ``unserialize()`` or ``igbinary_unserialize()``.
+
+In addition, there are some minor differences:
+
+* If the original variable defines them, all the semantics associated with
+  ``serialize()`` (such as ``__wakeup()``, ``__sleep()``, and ``Serializable``)
+  are preserved (``var_export()`` ignores them);
+* References involving ``SplObjectStorage``, ``ArrayObject`` or ``ArrayIterator``
+  instances are preserved;
+* Missing classes throw a ``ClassNotFoundException`` instead of being
+  unserialized to ``PHP_Incomplete_Class`` objects;
+* ``Reflection*``, ``IteratorIterator`` and ``RecursiveIteratorIterator``
+  classes throw an exception when being serialized.
+
+The exported data is a `PSR-2`_ compatible PHP file. Consider for example the
+following class hierarchy::
+
+    abstract class AbstractClass
+    {
+        protected $foo;
+        private $bar;
+
+        protected function setBar($bar)
+        {
+            $this->bar = $bar;
+        }
+    }
+
+    class ConcreteClass extends AbstractClass
+    {
+        public function __construct()
+        {
+            $this->foo = 123;
+            $this->setBar(234);
+        }
+    }
+
+When exporting the ``ConcreteClass`` data with VarExporter, the generated PHP
+file looks like this::
+
+    <?php
+    return \Symfony\Component\VarExporter\Internal\Hydrator::hydrate(
+        $o = [
+            clone (\Symfony\Component\VarExporter\Internal\Registry::$prototypes['Symfony\\Component\\VarExporter\\Tests\\ConcreteClass'] ?? \Symfony\Component\VarExporter\Internal\Registry::p('Symfony\\Component\\VarExporter\\Tests\\ConcreteClass')),
+        ],
+        null,
+        [
+            'Symfony\\Component\\VarExporter\\Tests\\AbstractClass' => [
+                'foo' => [
+                    123,
+                ],
+                'bar' => [
+                    234,
+                ],
+            ],
+        ],
+        $o[0],
+        []
+    );
+
+Instantiating PHP Classes
+-------------------------
+
+The other main feature provided by this component is an instantiator which can
+create objects and set their properties without calling their constructors or
+any other methods::
+
+    use Symfony\Component\VarExporter\Instantiator;
+
+    // creates an empty instance of Foo
+    $fooObject = Instantiator::instantiate(Foo::class);
+
+    // creates a Foo instance and sets one of its properties
+    $fooObject = Instantiator::instantiate(Foo::class, ['propertyName' => $propertyValue]);
+
+    // creates a Foo instance and sets a private property defined on its parent Bar class
+    $fooObject = Instantiator::instantiate(Foo::class, [], [
+        Bar::class => ['privateBarProperty' => $propertyValue],
+    ]);
+
+Instances of ``ArrayObject``, ``ArrayIterator`` and ``SplObjectHash`` can be
+created by using the special ``"\0"`` property name to define their internal value::
+
+    // Creates an SplObjectHash where $info1 is associated to $object1, etc.
+    $theObject = Instantiator::instantiate(SplObjectStorage::class, [
+        "\0" => [$object1, $info1, $object2, $info2...]
+    ]);
+
+    // creates an ArrayObject populated with $inputArray
+    $theObject = Instantiator::instantiate(ArrayObject::class, [
+        "\0" => [$inputArray]
+    ]);
+
+.. _`OPCache`: https://php.net/opcache
+.. _`PSR-2`: https://www.php-fig.org/psr/psr-2/