The JSON extension is bundled and compiled into PHP by default.
As of PHP 8.0.0, the JSON extension is a core PHP extension, so it is always enabled.
Information for installing this PECL extension may be found in the manual chapter titled Installation of PECL extensions. Additional information such as new releases, downloads, source files, maintainer information, and a CHANGELOG, can be located here: » https://pecl.php.net/package/json