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
-------

376cc03 don't override internal PHP constants
compare version using PHP_VERSION_ID

To let opcode caches optimize cached code, the `PHP_VERSION_ID`
constant is used to detect the current PHP version instead of calling
`version_compare()` with `PHP_VERSION`.
be smarter when guessing the document root
Merge branch '2.3' into 2.5

* 2.3:
  [SecurityBundle] Authentication entry point is only registered with firewall exception listener, not with authentication listeners
  be smarter when guessing the document root
  Azerbaijani locale
  Fixed grammar error in docblock
  Adjust upgrade file rendering
  [Bridge/Propel1] Changed deps to accepts all upcoming propel1 versions
  compare version using PHP_VERSION_ID
  [Form] Add doc for FormEvents
  don't override internal PHP constants

Conflicts:
	UPGRADE-3.0.md
	src/Symfony/Bundle/FrameworkBundle/Command/ServerRunCommand.php
	src/Symfony/Component/Debug/ErrorHandler.php
	src/Symfony/Component/HttpFoundation/Response.php