You can find more information about the core language API, here: http://api.haxe.org/php/
Read more about Haxe and PHP in the manual: http://haxe.org/manual/target-php.html
PHP itself is beyond the scope of this resource. Its assumed that you already are familiar with it, and you are here to learn about the Haxe PHP target.
The following resources are worth checking out as a refresher:
All you learn from these resources about PHP is applicable here!