Check if PHP extension is built-in or needs to be installed via PECL

Visit pecl.php.net and search for the package:

A package is built-in if you get no search results:

Or if when you click on the search result for the package, you see a message like this:

A package needs to be installed via PECL if you get a search result and when you click through it it, you don’t get the message about the package having been superseded: