Opening files for write from php modules

I'm trying to write an extention that calls a C++ lib i've written. 
I've gotten the module to compile and function mostly correctly, but I 
have a problem opening files for writing within functions of the lib. 
My code links libpng to write image files to a img directory on the 
webserver.  The problem is that any attempt to actually open a file for 
write fails:


this is the code to open the file (standard stuff):

FILE *pngout
pngout = fopen(filename, "wb");

if (pngout==NULL) {
         fprintf(stderr,"ERROR: Unable to open file %s for writing 
graph\n",filename);
         return -1;
}

This code works when i compile the lib as a stand-alone app, but fails 
when I call it from php.  I doubt permissions are the issue, I've gone 
as far as to chmod 777 the img dir it writes to, and I still get this 
error.  Is the problem that I need to use php streams to write to disk 
or something?  I'd like to be able to use the same lib linked into a php 
module and into a stand alone app.
Thanks.



Platform: PowerPC AIX 4.3.3. ml10
Compiler: xlC  (the C++ one)
PHP version: 4.0.6 (this is because this was the most resent IBM rpm for 
aix.  I had compile problems with newer versions, though i think b/c of 
the changes i've made to libtool, it should work now.) I had to edit 
libtool to use xlC's makeC++SharedLib script to get the module to link 
correctly.  I've compiled everything from my library to php to apache 
with xlC.