Fix typos and code examples in "PHP Extended"

I updated some of the code examples:
* There is no need to check for the POST method when using the `handleRequest`method. (http://symfony.com/doc/current/book/forms.html#book-form-handling-form-submissions)
* The `getRequest` method of the base `Controller` class has been deprecated since Symfony 2.4. (https://github.com/symfony/symfony/blob/master/UPGRADE-3.0.md#frameworkbundle)