constant
1.12.1
constant now accepts object instances as the second argument.
1.28
Using constant with the defined test was added in Twig 1.28.
constant returns the constant value for a given string:
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{{ some_date|date(constant('DATE_W3C')) }}
{{ constant('Namespace\\Classname::CONSTANT_NAME') }}
As of 1.12.1 you can read constants from object instances as well:
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{{ constant('RSS', date) }}
Use the defined test to check if a constant is defined:
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{% if constant('SOME_CONST') is defined %}
...
{% endif %}