@ostrolucky did a great job adding support for PHP constants in:

• Routing: [Routing] Parse PHP constants in YAML routing files symfony#25293
• Translation: [Translation] Parse PHP constants in YAML translation files symfony#25295
• Serializer: [Serializer] Parse PHP constants in YAML mappings symfony#25294

Let's review the docs to see if somewhere we mention that PHP constants are not supported. Also, let's think if we should add some note/tip about this or even some example somewhere.