The YAML Format

The YAML Format

According to the official YAML website, YAML is "a human friendly data serialization standard for all programming languages".

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.

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 (mappings).

Scalars

The syntax for scalars is similar to the PHP syntax.

Strings

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A string in YAML
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'A singled-quoted string in YAML'

Tip

In a single quoted string, a single quote ' must be doubled:

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'A single quote '' in a single-quoted string'
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"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:

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|
  \/ /| |\/| |
  / / | |  | |__

Alternatively, strings can be written with the folded style, denoted by >, where each line break is replaced by a space:

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>
  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

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# an integer
12
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# an octal
014
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# an hexadecimal
0xC
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# a float
13.4
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# an exponential number
1.2e+34
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# infinity
.inf

Nulls

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:

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2001-12-14t21:59:43.10-05:00
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# simple date
2002-12-14

Collections

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:

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- PHP
- Perl
- Python

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

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

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

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PHP: 5.2
MySQL: 5.1
Apache: 2.2.20

which is equivalent to this PHP code:

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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:

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PHP:    5.2
MySQL:  5.1
Apache: 2.2.20

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

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"symfony 1.0":
  PHP:    5.0
  Propel: 1.2
"symfony 1.2":
  PHP:    5.2
  Propel: 1.3

The following YAML is equivalent to the following PHP code:

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array(
  'symfony 1.0' => array(
    'PHP'    => 5.0,
    'Propel' => 1.2,
  ),
  'symfony 1.2' => array(
    'PHP'    => 5.2,
    'Propel' => 1.3,
  ),
);

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 tabulations.

You can nest sequences and mappings as you like:

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'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 ([]):

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[PHP, Perl, Python]

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

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

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

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'Chapter 1': [Introduction, Event Types]
'Chapter 2': [Introduction, Helpers]
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"symfony 1.0": { PHP: 5.0, Propel: 1.2 }
"symfony 1.2": { PHP: 5.2, Propel: 1.3 }

Comments

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

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# Comment on a line
"symfony 1.0": { PHP: 5.0, Propel: 1.2 } # Comment at the end of a line
"symfony 1.2": { PHP: 5.2, Propel: 1.3 }

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.

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