fopen() fails to open stream: permission denied, yet permissions should be valid

Question

So, I have this error:

Warning: fopen(/path/to/test-in.txt) [function.fopen]: failed to open stream: Permission denied

Performing ls

Answer 1

Another reason of this error may be that the directory of file does not exist.

I just add php code before fopen:

if(!file_exists(dirname($file))) 
    mkdir(dirname($file));

This help me out.