don't override internal PHP constants
minor #12372 [Yaml] don't override internal PHP constants (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Yaml] don't override internal PHP constants

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11783
| License       | MIT
| Doc PR        |

Commits
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376cc03 don't override internal PHP constants