The answer is:
$currentURL = $this->url();
I also asked this question here
Thanks to @jaruzafa
From the source code, I'd say it's getCurrentURL()
https://github.com/facebook/php-webdriver/blob/787e71db74e42cdf13a41d500f75ea43da84bc75/lib/WebDriver.php#L43
You can also use
$url=$this->getLocation();