Yaml component usage for Symfony2

.. index::
single: YAML
single: Configuration; YAML


YAML_ website is "a human friendly data serialization standard for all
programming languages". YAML is a simple language that describes data. As PHP,
it has a syntax for simple types like strings, booleans, floats, or integers.
But unlike PHP, it makes a difference between arrays (sequences) and hashes

The Symfony2 :namespace:Symfony\\Component\\Yaml Component knows how to
parse YAML and dump a PHP array to YAML.

.. note::

Even if the YAML format can describe complex nested data structure, this
chapter only describes the minimum set of features needed to use YAML as a
configuration file format.

Reading YAML Files

The :method:Symfony\\Component\\Yaml\\Parser::parse method parses a YAML
string and converts it to a PHP array::

use Symfony\Component\Yaml\Parser;
$yaml = new Parser();
$value = $yaml->parse(file_get_contents('/path/to/file.yaml'));

If an error occurs during parsing, the parser throws an exception indicating
the error type and the line in the original YAML string where the error

try {
 $value = $yaml->parse(file_get_contents('/path/to/file.yaml'));
} catch (\InvalidArgumentException $e) {
 // an error occurred during parsing
 echo "Unable to parse the YAML string: ".$e->getMessage();

.. tip::

As the parser is reentrant, you can use the same parser object to load
different YAML strings.

When loading a YAML file, it is sometimes better to use the
:method:Symfony\\Component\\Yaml\\Yaml::load wrapper method::

use Symfony\Component\Yaml\Yaml;
$loader = Yaml::load('/path/to/file.yml');

The Yaml::load() static method takes a YAML string or a file containing
YAML. Internally, it calls the Parser::parse() method, but with some added

  • It executes the YAML file as if it was a PHP file, so that you can embed
    PHP commands in YAML files;

  • When a file cannot be parsed, it automatically adds the file name to the
    error message, simplifying debugging when your application is loading
    several YAML files.

Writing YAML Files

The :method:Symfony\\Component\\Yaml\\Dumper::dump method dumps any PHP array
to its YAML representation::

use Symfony\Component\Yaml\Dumper;
$array = array('foo' => 'bar', 'bar' => array('foo' => 'bar', 'bar' => 'baz'));
$dumper = new Dumper();
$yaml = $dumper->dump($array);
file_put_contents('/path/to/file.yaml', $yaml);

.. note::

There are some limitations: the dumper is not able to dump resources and
dumping PHP objects is considered an alpha feature.

If you only need to dump one array, you can use the
:method:Symfony\\Component\\Yaml\\Yaml::dump static method shortcut::

$yaml = Yaml::dump($array, $inline);

The YAML format supports the two YAML array representations. By default, the
dumper uses the inline representation:

.. code-block:: yaml

{ foo: bar, bar: { foo: bar, bar: baz } }

But the second argument of the dump() method customizes the level at which
the output switches from the expanded representation to the inline one::

echo $dumper->dump($array, 1);

.. code-block:: yaml

foo: bar
bar: { foo: bar, bar: baz }

.. code-block:: php

echo $dumper->dump($array, 2);

.. code-block:: yaml

foo: bar
 foo: bar
 bar: baz

The YAML Syntax


.. code-block:: yaml A string in YAML .. code-block:: yaml 'A singled-quoted string in YAML' .. tip:: In a single quoted string, a single quote ``'`` must be doubled: .. code-block:: yaml 'A single quote '' in a single-quoted string' .. code-block:: yaml "A double-quoted string in YAML\n" Quoted styles are useful when a string starts or ends with one or more relevant spaces. .. tip:: The double-quoted style provides a way to express arbitrary strings, by using ``\`` escape sequences. It is very useful when you need to embed a ``\n`` or a unicode character in a string. When a string contains line breaks, you can use the literal style, indicated by the pipe (``|``), to indicate that the string will span several lines. In literals, newlines are preserved: .. code-block:: yaml | \/ /| |\/| | / / | | | |__ Alternatively, strings can be written with the folded style, denoted by ``>``, where each line break is replaced by a space: .. code-block:: yaml > This is a very long sentence that spans several lines in the YAML but which will be rendered as a string without carriage returns. .. note:: Notice the two spaces before each line in the previous examples. They won't appear in the resulting PHP strings. Numbers

.. code-block:: yaml

# an integer

.. code-block:: yaml

# an octal

.. code-block:: yaml

# an hexadecimal

.. code-block:: yaml

# a float

.. code-block:: yaml

# an exponential number

.. code-block:: yaml

# infinity


Nulls in YAML can be expressed with ``null`` or ``~``. Booleans ~~~~~~~~ Booleans in YAML are expressed with ``true`` and ``false``. Dates

YAML uses the ISO-8601 standard to express dates:

.. code-block:: yaml


.. code-block:: yaml

# simple date


A YAML file is rarely used to describe a simple scalar. Most of the time, it
describes a collection. A collection can be a sequence or a mapping of
elements. Both sequences and mappings are converted to PHP arrays.

Sequences use a dash followed by a space (- ):

.. code-block:: yaml

- Perl
- Python

The previous YAML file is equivalent to the following PHP code::

array('PHP', 'Perl', 'Python');

Mappings use a colon followed by a space (: ) to mark each key/value pair:

.. code-block:: yaml

PHP: 5.2
MySQL: 5.1
Apache: 2.2.20

which is equivalent to this PHP code::

array('PHP' => 5.2, 'MySQL' => 5.1, 'Apache' => '2.2.20');

.. note::

In a mapping, a key can be any valid scalar.

The number of spaces between the colon and the value does not matter:

.. code-block:: yaml

PHP: 5.2
MySQL: 5.1
Apache: 2.2.20

YAML uses indentation with one or more spaces to describe nested collections:

.. code-block:: yaml

"symfony 1.4":
 PHP: 5.2
 Doctrine: 1.2
 PHP: 5.3
 Doctrine: 2.0

The following YAML is equivalent to the following PHP code::

 'symfony 1.4' => array(
 'PHP' => 5.2,
 'Doctrine' => 1.2,
 'Symfony2' => array(
 'PHP' => 5.3,
 'Doctrine' => 2.0,

There is one important thing you need to remember when using indentation in a
YAML file: Indentation must be done with one or more spaces, but never with

You can nest sequences and mappings as you like:

.. code-block:: yaml

'Chapter 1':
 - Introduction
 - Event Types
'Chapter 2':
 - Introduction
 - Helpers

YAML can also use flow styles for collections, using explicit indicators
rather than indentation to denote scope.

A sequence can be written as a comma separated list within square brackets

.. code-block:: yaml

[PHP, Perl, Python]

A mapping can be written as a comma separated list of key/values within curly
braces ({}):

.. code-block:: yaml

{ PHP: 5.2, MySQL: 5.1, Apache: 2.2.20 }

You can mix and match styles to achieve a better readability:

.. code-block:: yaml

'Chapter 1': [Introduction, Event Types]
'Chapter 2': [Introduction, Helpers]

.. code-block:: yaml

"symfony 1.4": { PHP: 5.2, Doctrine: 1.2 }
"Symfony2": { PHP: 5.3, Doctrine: 2.0 }


Comments can be added in YAML by prefixing them with a hash mark (#):

.. code-block:: yaml

# Comment on a line
"Symfony2": { PHP: 5.3, Doctrine: 2.0 } # Comment at the end of a line

.. note::

Comments are simply ignored by the YAML parser and do not need to be
indented according to the current level of nesting in a collection.

Dynamic YAML files

In Symfony2, a YAML file can contain PHP code that is evaluated just before the
parsing occurs:

.. code-block:: yaml

 version: <?php echo file_get_contents('1.0/VERSION')."\n" ?>
 version: "<?php echo file_get_contents('1.1/VERSION') ?>"

Be careful to not mess up with the indentation. Keep in mind the following
simple tips when adding PHP code to a YAML file:

  • The <?php ?> statements must always start the line or be embedded in a

  • If a <?php ?> statement ends a line, you need to explicitly output a new
    line ("\n").

.. _YAML: http://yaml.org/


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