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
----------

[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`.
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
Merge branch '2.5' into 2.6

* 2.5:
  [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
  [Session] Fix parameter names in WriteCheckSessionHandler
  Add consistency with request type checking
  [FrameworkBundle] Fix server run in case the router script does not exist

Conflicts:
	composer.json
	src/Symfony/Component/Debug/ErrorHandler.php
	src/Symfony/Component/Debug/ExceptionHandler.php
Merge branch '2.6' into 2.7

* 2.6:
  Extract an AbstractEventDispatcherTest from EventDispatcherTest and also use it in ContainerAwareEventDispatcherTest
  [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
  [HttpKernel] fix parse error in DumpDataCollector
  [TwigBundle/DebugBundle] move dump extension & cleanups
  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
  *_timezone changes also affect the BirthdayType
  don't override internal PHP constants
  Drop support for model_timezone and view_timezone options in TimeType and DateType.
  [DomCrawler] Added support for link tags in the Link class
  [Session] Fix parameter names in WriteCheckSessionHandler
  Add consistency with request type checking
  [FrameworkBundle] Fix server run in case the router script does not exist
Merge branch '2.7'

* 2.7:
  Extract an AbstractEventDispatcherTest from EventDispatcherTest and also use it in ContainerAwareEventDispatcherTest
  [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
  [HttpKernel] fix parse error in DumpDataCollector
  [TwigBundle/DebugBundle] move dump extension & cleanups
  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
  *_timezone changes also affect the BirthdayType
  don't override internal PHP constants
  Drop support for model_timezone and view_timezone options in TimeType and DateType.
  [DomCrawler] Added support for link tags in the Link class
  [Session] Fix parameter names in WriteCheckSessionHandler
  Add consistency with request type checking
  [FrameworkBundle] Fix server run in case the router script does not exist