This is how I got UI to work on my 64-bit Windows 10 local computer:
Step 1: I went to https://pecl.php.net/package/ui and selected the v2.0.0 DLL for Windows
Step 2: I downloaded the version 2.0.0 zipfile for TS and x64
Step 3: I created a folder for the extracted files at C:\php_ext
Step 4: I had DLL-location-related issues with my current version 8.3.11. So I downloaded PHP 7.1.7 from https://windows.php.net/downloads/releases/archives/. 
Step 5: I extracted the contents from the zipfile and installed in a separate folder called C:\php7.1.7
Step 6: Inside Windows=>Control Panel=>System=>Advanced system settings=>Environment Variables, I added C:\php7.1.7 and C:\php_ext into the Administrator Path environment variable
Step 7: I also added C:\php7.1.7 and C:\php_ext into the System variables Path environment variable
Step 8: I copied C:\php7.1.7\php.ini-production to C:\php7.1.7\php.ini
Step 9: In C:\php7.1.7\php.ini I added the entry: extension_dir = "C:\php_ext"
Step 10. In C:\php7.1.7\php.ini I added the entry: extension=php_ui.dll
Step 11. I ran php --ri ui. It outputted: ui support => enabled
Step 12: I ran "php C:\php_ext\gallery.php" and the gallery Windows GUI app worked fine!

Hello, PHP community!

The extension works, but weird things happen.

Go to PECL and download the 2.0.0 DLL package for Windows and the 7.1.33 PHP version.

Configuring the language, so it points to its own ext directory and putting there the UI library DLL is the only thing you have to do.

Then just open the examples which come with the library DLL tar archive. It works!

No support in PhpStorm and the loading of the library somehow throws an error, but it works!