The PHP native JSON extension has been bundled and compiled with PHP by default since version 5.2.0. Because the minimum version of PHP required by WordPress is now 5.6.20 (see #46594 and [45058]), the related polyfills and workarounds have been removed (see [46205-46206,46208]).
However, even though the JSON extension is now included in PHP by default, it is still possible to disable the extension in a custom configuration. This change will prevent sites from upgrading if the JSON extension is disabled to prevent compatibility issues.
Props jrf, jorbin, dd32, desrosj.
Fixes#47699.
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