PHP keywords in XML

By default, ``true``, ``false`` and ``null`` in XML are converted to the PHP

keywords (respectively ``true``, ``false`` and ``null``):

..code-block::xml

To disable this behaviour, use the ``string`` type:

..code-block::xml

..note::

This is not available for Yaml and PHP, because they already have built-in

support for the PHP keywords.

Referencing Services with Parameters

A parameter can also reference to a service. While doing so, it specifies an

invalid behaviour.

Yaml

Start the string with ``@``, ``@@`` or ``@?`` to reference a service in Yaml.

* ``@mailer`` references to the ``mailer`` service. If the service does not

exists, an exception will be thrown;

* ``@?mailer`` references to the ``mailer`` service. If the service does not

exists, it will be ignored;

Xml

In XML, use the ``service`` type. The behaviour if the service does not exists

can be specified using the ``on-invalid`` argument (it can be set to ``null``

to return ``null`` or ``ignored`` to let the container ignore the error, if

not specified it throws an exception).

Php

In PHP, you can use the

:class:`Symfony\\Component\\DependencyInjection\\Reference` class to reference

a service.