The Yaml Component
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The Yaml Component
The Yaml component loads and dumps YAML files.
What is It?
The Symfony Yaml component parses YAML strings to convert them to PHP arrays. It is also able to convert PHP arrays to YAML strings.
YAML, YAML Ain't Markup Language, is a human friendly data serialization standard for all programming languages. YAML is a great format for your configuration files. YAML files are as expressive as XML files and as readable as INI files.
The Symfony Yaml Component implements a selected subset of features defined in the YAML 1.2 version specification.
Tip
Learn more about the Yaml component in the The YAML Format article.
Installation
You can install the component in 2 different ways:
- Install it via Composer (
symfony/yaml
on Packagist); - Use the official Git repository (https://github.com/symfony/yaml).
Then, require the vendor/autoload.php
file to enable the autoloading mechanism
provided by Composer. Otherwise, your application won't be able to find the classes
of this Symfony component.
Why?
Fast
One of the goals of Symfony Yaml is to find the right balance between speed and features. It supports just the needed features to handle configuration files. Notable lacking features are: document directives, multi-line quoted messages, compact block collections and multi-document files.
Real Parser
It sports a real parser and is able to parse a large subset of the YAML specification, for all your configuration needs. It also means that the parser is pretty robust, easy to understand, and simple enough to extend.
Clear Error Messages
Whenever you have a syntax problem with your YAML files, the library outputs a helpful message with the filename and the line number where the problem occurred. It eases the debugging a lot.
Dump Support
It is also able to dump PHP arrays to YAML with object support, and inline level configuration for pretty outputs.
Types Support
It supports most of the YAML built-in types like dates, integers, octals, booleans, and much more...
Full Merge Key Support
Full support for references, aliases, and full merge key. Don't repeat yourself by referencing common configuration bits.
Using the Symfony YAML Component
The Symfony Yaml component is very simple and consists of two main classes: one parses YAML strings (Parser), and the other dumps a PHP array to a YAML string (Dumper).
On top of these two classes, the Yaml class acts as a thin wrapper that simplifies common uses.
Reading YAML Files
The parse() method parses a YAML string and converts it to a PHP array:
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use Symfony\Component\Yaml\Yaml;
$value = Yaml::parse(file_get_contents('/path/to/file.yml'));
Caution
Because it is currently possible to pass a filename to this method, you must validate the input first. Passing a filename is deprecated in Symfony 2.2, and was removed in Symfony 3.0.
If an error occurs during parsing, the parser throws a ParseException exception indicating the error type and the line in the original YAML string where the error occurred:
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use Symfony\Component\Yaml\Exception\ParseException;
try {
$value = Yaml::parse(file_get_contents('/path/to/file.yml'));
} catch (ParseException $e) {
printf("Unable to parse the YAML string: %s", $e->getMessage());
}
Writing YAML Files
The dump() method dumps any PHP array to its YAML representation:
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use Symfony\Component\Yaml\Yaml;
$array = array(
'foo' => 'bar',
'bar' => array('foo' => 'bar', 'bar' => 'baz'),
);
$yaml = Yaml::dump($array);
file_put_contents('/path/to/file.yml', $yaml);
If an error occurs during the dump, the parser throws a DumpException exception.
Array Expansion and Inlining
The YAML format supports two kind of representation for arrays, the expanded one, and the inline one. By default, the dumper uses the expanded representation:
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foo: bar
bar:
foo: bar
bar: baz
The second argument of the dump() method customizes the level at which the output switches from the expanded representation to the inline one:
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echo Yaml::dump($array, 1);
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foo: bar
bar: { foo: bar, bar: baz }
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echo Yaml::dump($array, 2);
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foo: bar
bar:
foo: bar
bar: baz
Indentation
By default the YAML component will use 4 spaces for indentation. This can be changed using the third argument as follows:
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// use 8 spaces for indentation
echo Yaml::dump($array, 2, 8);
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foo: bar
bar:
foo: bar
bar: baz
Numeric Literals
3.2
Support for parsing integers grouped by underscores was introduced in Symfony 3.2.
Long numeric literals, being integer, float or hexadecimal, are known for their poor readability in code and configuration files. That's why YAML files allow to add underscores to improve their readability:
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parameters:
credit_card_number: 1234_5678_9012_3456
long_number: 10_000_000_000
pi: 3.14159_26535_89793
hex_words: 0x_CAFE_F00D
During the parsing of the YAML contents, all the _
characters are removed
from the numeric literal contents, so there is not a limit in the number of
underscores you can include or the way you group contents.
Advanced Usage: Flags
3.1
Flags were introduced in Symfony 3.1 and replaced the earlier boolean arguments.
Object Parsing and Dumping
You can dump objects by using the DUMP_OBJECT
flag:
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$object = new \stdClass();
$object->foo = 'bar';
$dumped = Yaml::dump($object, 2, 4, Yaml::DUMP_OBJECT);
// !php/object:O:8:"stdClass":1:{s:5:"foo";s:7:"bar";}
And parse them by using the PARSE_OBJECT
flag:
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$parsed = Yaml::parse($dumped, Yaml::PARSE_OBJECT);
var_dump(is_object($parsed)); // true
echo $parsed->foo; // bar
The YAML component uses PHP's serialize()
method to generate a string
representation of the object.
Caution
Object serialization is specific to this implementation, other PHP YAML
parsers will likely not recognize the php/object
tag and non-PHP
implementations certainly won't - use with discretion!
Handling Invalid Types
By default the parser will encode invalid types as null
. You can make the
parser throw exceptions by using the PARSE_EXCEPTION_ON_INVALID_TYPE
flag:
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$yaml = '!php/object:O:8:"stdClass":1:{s:5:"foo";s:7:"bar";}';
Yaml::parse($yaml, Yaml::PARSE_EXCEPTION_ON_INVALID_TYPE); // throws an exception
Similarly you can use DUMP_EXCEPTION_ON_INVALID_TYPE
when dumping:
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$data = new \stdClass(); // by default objects are invalid.
Yaml::dump($data, 2, 4, Yaml::DUMP_EXCEPTION_ON_INVALID_TYPE); // throws an exception
echo $yaml; // { foo: bar }
Date Handling
By default the YAML parser will convert unquoted strings which look like a
date or a date-time into a Unix timestamp; for example 2016-05-27
or
2016-05-27T02:59:43.1Z
(ISO-8601):
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Yaml::parse('2016-05-27'); // 1464307200
You can make it convert to a DateTime
instance by using the PARSE_DATETIME
flag:
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$date = Yaml::parse('2016-05-27', Yaml::PARSE_DATETIME);
var_dump(get_class($date)); // DateTime
Dumping Multi-line Literal Blocks
In YAML multiple lines can be represented as literal blocks, by default the dumper will encode multiple lines as an inline string:
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$string = array("string" => "Multiple\nLine\nString");
$yaml = Yaml::dump($string);
echo $yaml; // string: "Multiple\nLine\nString"
You can make it use a literal block with the DUMP_MULTI_LINE_LITERAL_BLOCK
flag:
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$string = array("string" => "Multiple\nLine\nString");
$yaml = Yaml::dump($string, 2, 4, Yaml::DUMP_MULTI_LINE_LITERAL_BLOCK);
echo $yaml;
// string: |
// Multiple
// Line
// String
Parsing PHP Constants
By default, the YAML parser treats the PHP constants included in the contents as
regular strings. Use the PARSE_CONSTANT
flag and the special !php/const:
syntax to parse them as proper PHP constants:
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$yaml = '{ foo: PHP_INT_SIZE, bar: !php/const:PHP_INT_SIZE }';
$parameters = Yaml::parse($yaml, Yaml::PARSE_CONSTANT);
// $parameters = array('foo' => 'PHP_INT_SIZE', 'bar' => 8);
Syntax Validation
The syntax of YAML contents can be validated through the CLI using the LintCommand command.
First, install the Console component:
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$ composer require symfony/console
Create a console application with lint:yaml
as its only command:
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// lint.php
use Symfony\Component\Console\Application;
use Symfony\Component\Yaml\Command\LintCommand;
(new Application('yaml/lint'))
->add(new LintCommand())
->getApplication()
->setDefaultCommand('lint:yaml', true)
->run();
Then, execute the script for validating contents:
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# validates a single file
$ php lint.php path/to/file.yml
# or all the files in a directory
$ php lint.php path/to/directory
# or contents passed to STDIN
$ cat path/to/file.yml | php lint.php
The result is written to STDOUT and uses a plain text format by default.
Add the --format
option to get the output in JSON format:
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$ php lint.php path/to/file.yml --format json