Contributed by
Guilhem Niot
in #21194.

The YAML format is well known for being simple and readable, but under the hood it defines lots of advanced features for complex needs. One of those features are YAML tags, which are primarily used to define explicit data types.

The Symfony Yaml component already supports several tags, such as !!binary for storing binary data, and !php/const: for referring to PHP constants. In Symfony 3.3 you now can define your own YAML tags thanks to a new flag called Yaml::PARSE_CUSTOM_TAGS.

Custom tags are arbitrary strings that start with the ! character, such as the !my_tag value in the following example:

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!my_tag { foo: bar }

Use the following PHP code to parse it:

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use Symfony\Component\Yaml\Yaml;

$yaml = "!my_tag { foo: bar }";
$config = Yaml::parse($yaml, Yaml::PARSE_CUSTOM_TAGS);


$tag = $config->getTag();     // $tag = 'my_tag'
$value = $config->getValue(); // $value = array('foo' => 'bar')

The semantics of the YAML tags depend on each application, so the parser wraps those values in a generic Symfony\Component\Yaml\Tag\TaggedValue class. In the above example, the $config variable now contains:

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TaggedValue('my_tag', array('foo' => 'bar'));

The support for YAML tags goes both ways, so if your PHP code contains TaggedValue objects, they will be transformed into YAML tags:

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$config = array(
    new TaggedValue('foo', array('bar')),
    new TaggedValue('quz', array('baz')),
);

// The generated YAML code is:
// - !foo [bar]
// - !quz [baz]