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.