PHP is a modular system where you can install and enable new modules provide extra functionalities. These modules or extensions can also be disabled to enhance PHP's performance and reduce your security vector.
Shared hosting system such as cPanel doesn't let you install third party PHP extensions yourself but it normally comes with most standard extensions and you can enable or disable them from cPanel's dashboard.
Modules are enabled and disabled only for the selected version. You'll need to enable or disable the PHP modules again if you switch to another PHP version later.
Modules are immediately enabled or disabled when checkbox is ticked or unticked.
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