About Webapi 1
You started with a hello world, that's an example you can use on all platforms.
It's from the Haxe Generic API, available for all platforms. Now let's look at the ServerSide specific API, which is located in the package php.
This will use the php.Lib.print function that prints some raw string (without adding debug information). It will print the parameters sent by the browser.
There is also a lot of useful functionality available in the Web class. Read up on that here: http://api.haxe.org/php/Web.html
Note that the php package tries to mimic as far as possible the neko one. This allows for easy switching between the two platforms