PHP：在selenium测试中发送左，右，上，下箭头键

I am writing selenium browser tests in php. To test dropdown and HTML5 date inputs, I want to send the arrow keys to the browser, something like:

$dateInput->sendKeys(ARROW_LEFT);

I found quite some examples in Java, but none for PHP. I tried to find the ASCII code of these keys to send them with chr (ascii-number), but the arrow keys actually are not in the 128 ascii signs.

So, can anyone hint me in the right direction how to send the arrow keys in PHP?

Thanks a lot!