[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        |
This should be done outside of the function to avoid computing it each time a value is escaped.