Understanding YAML

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

Strings in YAML can be wrapped both in single and double quotes. In some cases, they can also be unquoted:

A string in YAML

'A singled-quoted string in YAML'

"A double-quoted string in YAML"

Quoted styles are useful when a string starts or end with one or more relevant spaces, because unquoted strings are trimmed on both end when parsing their contents. Quotes are required when the string contains special or reserved characters.

When using single-quoted strings, any single quote ' inside its contents must be doubled to escape it:

'A single quote '' inside a single-quoted string'

Strings containing any of the following characters must be quoted. Although you can use double quotes, for these characters it is more convenient to use single quotes, which avoids having to escape any backslash \:

• :, {, }, [, ], ,, &, *, #, ?, |, -, <, >, =, !, %, @, \

The double-quoted style provides a way to express arbitrary strings, by using \ to escape characters and sequences. For instance, it is very useful when you need to embed a \n or a Unicode character in a string.

"A double-quoted string in YAML\n"

If the string contains any of the following control characters, it must be escaped with double quotes:

• \0, \x01, \x02, \x03, \x04, \x05, \x06, \a, \b, \t, \n, \v, \f, \r, \x0e, \x0f, \x10, \x11, \x12, \x13, \x14, \x15, \x16, \x17, \x18, \x19, \x1a, \e, \x1c, \x1d, \x1e, \x1f, \N, \_, \L, \P

Finally, there are other cases when the strings must be quoted, no matter if you're using single or double quotes:

• When the string is true or false (otherwise, it would be treated as a boolean value);
• When the string is null or ~ (otherwise, it would be considered as a null value);
• When the string looks like a number, such as integers (e.g. 2, 14, etc.), floats (e.g. 2.6, 14.9) and exponential numbers (e.g. 12e7, etc.) (otherwise, it would be treated as a numeric value);
• When the string looks like a date (e.g. 2014-12-31) (otherwise it would be automatically converted into a Unix timestamp).

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:

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

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

>
This is a very long sentence
that spans several lines in the YAML
but which will be rendered as a string
without carriage returns.

Notice the two spaces before each line in the previous examples. They won't appear in the resulting PHP strings.

Numbers

# an integer
12

# an octal
014

# an hexadecimal
0xC

# a float
13.4

# an exponential number
1.2e+34

# 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:

2001-12-14t21:59:43.10-05:00

# 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:

- PHP
- 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:

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');

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

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

PHP:    5.2
MySQL:  5.1
Apache: 2.2.20

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

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

The above YAML is equivalent to the following PHP code:

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 tabulators.

You can nest sequences and mappings as you like:

'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 ([]):

[PHP, Perl, Python]

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

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

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

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

'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 (#):

# 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 }

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.

Taken from Symfony YAML doc