Add more native support for php constants in yaml files

[thedustin]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Documented?	yes
Fixed tickets	#303, #304
License	MIT

I would like to add "more native" support for PHP constants in YAML files by enabling the !php/const tag.

I struggled a bit while creating my first GraphQL project today, because I expected this feature to be available. Also this way the constants don‘t need to be fetched during runtime with the PHP function \constant().

This topic was already discussed in #303 but closed because of inactivity.

[mcg-web]

Thank you.