Do not move to www to keep buildpack compatible with other buildpacks

Hello,

I know this probably won't be merged, but I would like to raise the concern that the PHP buildpack is not compatible with [heroku-multi-buildpack](https://github.com/ddollar/heroku-buildpack-multi) because you move all the files to www and other buildpack do not detect properly and it also creates issues with other buildpacks `$PATH`.

I think a better solution might be to add an option to enable or disable the moving of all files to `www`, but for now, for those having an issue with multi buildpack setup & the php buildpack, you may use [heroku-buildpack-php-multi](https://github.com/hooktstudios/heroku-buildpack-php-multi) fork which simply do not move files to a new folder.

Have a nice day and happy coding!