Permission denied when opening or creating files with PHP

I
can't create files with php, because the file dosent got permission for
that.
I get this
error:

Warning: fopen(test.txt): failed to open stream: Permission denied in
/web/com/example.com/index.php on line 20
Warning: fwrite() expects parameter
1 to be resource, boolean given in /web/com/example.com/index.php on line 21 />Warning: fclose() expects parameter 1 to be resource, boolean given in
/web/com/example.com/index.php on line 22 />

This is the
code I was
using:

 $file =
            fopen("test.txt","w");
 echo fwrite($file,"Hello World. Testing!");

            fclose($file);

 ?>
            

Simple as that! This
is example code from
w3schools.

So I need help to find out
how to give the file the needed permissions.
I'm uploading files to my site
with the net2ftp
FTP web application, if it matters.