[DependencyInjection] Trim constants

[gnugat]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
License	MIT

When indenting constant parameters in XML configuration files, an exception is throwed.
This PR simply adds:

• a test for simple constant support in XML configuration files
• a test for indented constant support in XML configuration files (proving the issue)
• a fix by trimming constants

Example to reproduce the bug

Configuration sample:

<?xml version="1.0" ?>

<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">
  <parameters>
    <parameter key="simple_constant" type="constant">PHP_EOL</parameter>
    <parameter key="indented_constant" type="constant">
        PHP_EOL
    </parameter>
  </parameters>
</container>

Code sample:

<?php
$container = new ContainerBuilder();

$loader = new XmlFileLoader($container, new FileLocator(__DIR__);
$loader->load('indented_constant.xml');

$container->getParameterBag()->all();

Result:

constant(): Couldn't find constant 
        PHP_EOL

Notice the fact that the error message is not:

constant(): Couldn't find constant PHP_EOL

[wouterj]

This is already proposed multiple times and refused. The reason (see #4463 ):

The rule is simple enough: All characters are significant, including whitespace.
-- fabpot

[gnugat]

Then maybe the first commit, which adds a test for constant support, can still be useful?

I'd be curious to see cases where whitespaces are used for constant names.

[wouterj]

Whitespaces for constant names are not common, but we can't make an exception for some values. See also #3646

[gnugat]

@wouterj yes, I was looking at this PR, and also at its related issue #3644 and another PR #6123.

While I still don't understand why this behavior should be kept, I think I'm going to take another approach: I'll submit a PR to add a test on constants support and another one on the component documentation.

[wouterj]

👍 for both of those PR you're going to create. I missed the test commit and we also don't have this documented.

[stof]

@gnugat People may want to define some parameters with whitespaces in it. If we are trimming the value, it becomes impossible