Implement WeakReference "hack" for PHP < 7.4

[Sanchous98]

Using refcount value, provided by "debug_zval_dump", we can implement smth. similar to WeakReference from PHP 7.4.

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony polyfills repository. It is used by Composer to allow developers to depend on specific Symfony PHP polyfills.

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