symfony

This PR was merged into the 2.7 branch.

Discussion
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[HttpFoundation] Fix forward-compat of NativeSessionStorage with PHP 7.2

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #24524
| License       | MIT
| Doc PR        | ø

PHP 7.2 disallow setting session options when the session was already started. This PR will not set any option if the session is already started and throw an exception if trying to do so with custom options.

Commits
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00a1357d [HttpFoundation] Fix forward-compat of NativeSessionStorage with PHP 7.2