为什么intl_error_name返回未定义的函数？

I'm trying to call the PHP function intl_error_name() but it's returning a

Fatal error: Call to undefined function.

function ErrorHandler($errno , $errstr, $errfile, $errline, $errcontext)
{
    if (!(error_reporting() & $errno)) {
        return;
    }
    echo intl_error_name($errno);
}
set_error_handler("ErrorHandler");

trigger_error("Test");

The documentation seems pretty straightforward.

Do I need to enable any extensions in order to use this? I tried enabling extension=php_intl.dll but it didn't work. I'm using PHP version 5.5.12.

you first need to install the PHP library to use this function.

See: http://www.php.net/manual/en/intl.requirements.php