What is the reason for Composer (PHP) permitting me to mention "ext-json" but not other extensions such as "ext-gmp" while using "composer require"?

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The reason for this is that "ext-json" is a built-in extension in PHP and is available in all PHP installations. On the other hand, "ext-gmp" is not a built-in extension and therefore needs to be installed separately on the server before it can be used. Composer only allows the installation of packages that are available in the PHP installation by default or can be easily installed using Composer itself. If you want to use a non-default extension like "ext-gmp", you need to ensure that it is installed on your server before running the Composer command.