PHP in Windows: add an environment variable for User or Machine

I am trying to update the Path environment variable in Windows with a PHP script, but if I use putenv() it doesn’t change. I can get the paths list in the Path variable but I am not able to update it.
Another problem is that if I use getenv('Path') to get the paths list, I have a unique string with all paths merged from the Path variable of User and of the Machine.

I’m wondering if there is a better way to do that, maybe acting directly in the registry key to update User Path variable.