The Yaml Component

The Yaml Component

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

$ composer require symfony/yaml

Note

If you install this component outside of a Symfony application, you must require the vendor/autoload.php file in your code to enable the class autoloading mechanism provided by Composer. Read this article for more details.

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, octal numbers, 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 consists of two main classes: one parses YAML strings (Symfony\Component\Yaml\Parser), and the other dumps a PHP array to a YAML string (Symfony\Component\Yaml\Dumper).

On top of these two classes, the Symfony\Component\Yaml\Yaml class acts as a thin wrapper that simplifies common uses.

Reading YAML Contents

The parse() method parses a YAML string and converts it to a PHP array:

use Symfony\Component\Yaml\Yaml;
$value = Yaml::parse("foo: bar");
// $value = ['foo' => 'bar']

If an error occurs during parsing, the parser throws a Symfony\Component\Yaml\Exception\ParseException exception indicating the error type and the line in the original YAML string where the error occurred:

use Symfony\Component\Yaml\Exception\ParseException;
try {
    $value = Yaml::parse('...');
} catch (ParseException $exception) {
    printf('Unable to parse the YAML string: %s', $exception->getMessage());
}

Reading YAML Files

The parseFile() method parses the YAML contents of the given file path and converts them to a PHP value:

use Symfony\Component\Yaml\Yaml;
$value = Yaml::parseFile('/path/to/file.yaml');

If an error occurs during parsing, the parser throws a ParseException exception.

Writing YAML Files

The dump() method dumps any PHP array to its YAML representation:

use Symfony\Component\Yaml\Yaml;
$array = [
    'foo' => 'bar',
    'bar' => ['foo' => 'bar', 'bar' => 'baz'],
];
$yaml = Yaml::dump($array);
file_put_contents('/path/to/file.yaml', $yaml);

If an error occurs during the dump, the parser throws a Symfony\Component\Yaml\Exception\DumpException exception.

Expanded and Inlined Arrays

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:

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:

echo Yaml::dump($array, 1);

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

echo Yaml::dump($array, 2);

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:

// uses 8 spaces for indentation
echo Yaml::dump($array, 2, 8);

foo: bar
bar:
        foo: bar
        bar: baz

Numeric Literals

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:

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

Object Parsing and Dumping

You can dump objects by using the DUMP_OBJECT flag:

$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:

$parsed = Yaml::parse($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!

Parsing and Dumping Objects as Maps

You can dump objects as Yaml maps by using the DUMP_OBJECT_AS_MAP flag:

$object = new \stdClass();
$object->foo = 'bar';
$dumped = Yaml::dump(['data' => $object], 2, 4, Yaml::DUMP_OBJECT_AS_MAP);
// $dumped = "data:\n    foo: bar"

And parse them by using the PARSE_OBJECT_FOR_MAP flag:

$parsed = Yaml::parse($dumped, Yaml::PARSE_OBJECT_FOR_MAP);
var_dump(is_object($parsed)); // true
var_dump(is_object($parsed->data)); // true
echo $parsed->data->foo; // bar

The YAML component uses PHP’s `(array) casting to generate a string representation of the object as a map.

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:

$yaml = '!php/object \'O:8:"stdClass":1:{s:5:"foo";s:7:"bar";}\'';
Yaml::parse($yaml, Yaml::PARSE_EXCEPTION_ON_INVALID_TYPE); // throws an exception

Similarly you can use DUMP_EXCEPTION_ON_INVALID_TYPE when dumping:

$data = new \stdClass(); // by default objects are invalid.
Yaml::dump($data, 2, 4, Yaml::DUMP_EXCEPTION_ON_INVALID_TYPE); // throws an exception

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):

Yaml::parse('2016-05-27'); // 1464307200

You can make it convert to a DateTime instance by using the PARSE_DATETIME flag:

$date = Yaml::parse('2016-05-27', Yaml::PARSE_DATETIME);
var_dump(get_class($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:

$string = ["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:

$string = ["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:

$yaml = '{ foo: PHP_INT_SIZE, bar: !php/const PHP_INT_SIZE }';
$parameters = Yaml::parse($yaml, Yaml::PARSE_CONSTANT);
// $parameters = ['foo' => 'PHP_INT_SIZE', 'bar' => 8];

Parsing and Dumping of Binary Data

You can dump binary data by using the DUMP_BASE64_BINARY_DATA flag:

$imageContents = file_get_contents(__DIR__.'/images/logo.png');
$dumped = Yaml::dump(['logo' => $imageContents], 2, 4, Yaml::DUMP_BASE64_BINARY_DATA);
// logo: !!binary iVBORw0KGgoAAAANSUhEUgAAA6oAAADqCAY...

Binary data is automatically parsed if they include the !!binary YAML tag (there’s no need to pass any flag to the Yaml parser):

$dumped = 'logo: !!binary iVBORw0KGgoAAAANSUhEUgAAA6oAAADqCAY...';
$parsed = Yaml::parse($dumped);
$imageContents = $parsed['logo'];

Parsing and Dumping Custom Tags

In addition to the built-in support of tags like !php/const and !!binary, you can define your own custom YAML tags and parse them with the PARSE_CUSTOM_TAGS flag:

$data = "!my_tag { foo: bar }";
$parsed = Yaml::parse($data, Yaml::PARSE_CUSTOM_TAGS);
// $parsed = Symfony\Component\Yaml\Tag\TaggedValue('my_tag', ['foo' => 'bar']);
$tagName = $parsed->getTag();    // $tagName = 'my_tag'
$tagValue = $parsed->getValue(); // $tagValue = ['foo' => 'bar']

If the contents to dump contain Symfony\Component\Yaml\Tag\TaggedValue objects, they are automatically transformed into YAML tags:

use Symfony\Component\Yaml\Tag\TaggedValue;
$data = new TaggedValue('my_tag', ['foo' => 'bar']);
$dumped = Yaml::dump($data);
// $dumped = '!my_tag { foo: bar }'

Dumping Null Values

The official YAML specification uses both null and ~ to represent null values. This component uses null by default when dumping null values but you can dump them as ~ with the DUMP_NULL_AS_TILDE flag:

$dumped = Yaml::dump(['foo' => null]);
// foo: null
$dumped = Yaml::dump(['foo' => null], 2, 4, Yaml::DUMP_NULL_AS_TILDE);
// foo: ~

Syntax Validation

The syntax of YAML contents can be validated through the CLI using the Symfony\Component\Yaml\Command\LintCommand command.

First, install the Console component:

$ composer require symfony/console

Create a console application with lint:yaml as its only command:

// 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:

# validates a single file
$ php lint.php path/to/file.yaml
# or validates multiple files
$ php lint.php path/to/file1.yaml path/to/file2.yaml
# or all the files in a directory
$ php lint.php path/to/directory
# or all the files in multiple directories
$ php lint.php path/to/directory1 path/to/directory2
# or contents passed to STDIN
$ cat path/to/file.yaml | 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:

$ php lint.php path/to/file.yaml --format json

Tip

The linting command will also report any deprecations in the checked YAML files. This may for example be useful for recognizing deprecations of contents of YAML files during automated tests.

Learn More

The YAML Format

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.