fopen problem

I am having problems with fopen.

A php file in the app directory but not using wordpress it works accessing /data/m.csv

The same code in a plugin accessing that data in the https://myapp/data directory above dosent work

The same code in a plugin accessing that data in the wp-includes directory (using the fopen includes flag =true) dosent work.

The same code in a plugin accessing that data in a plugin/myplugin/data directory dosent work.

Any help much appreciated