To build the extension you need to install the » ICU library, version 4.0.0 or newer is required. As of PHP 7.4.0 ICU 50.1 or newer is required.
This extension is bundled with PHP as of PHP version 5.3.0. Alternatively, the PECL version of this extension may be used with all PHP versions greater than 5.2.0 (5.2.4+ recommended).