helper -> call to undefined function

Yes, the file name is the same in the code and on the file.
I even renamed the file to form_helper_php, which is a native helper of CI, in order to "extend it", as described in the documentation. It didn't work.

One thing to consider:

My localhost server is Windows with XAMPP.

The host I uploaded is Linux with Apache.

Could it be something related to file permission? I'll check on it and let you guys know it.