www-data runs in terminal but in php it does not execute, apache2, debian squeeze

www-data runs in terminal but in php it does not execute, apache2, debian
squeeze

I have external program.exe inside my website that gets data from text
file, process it and then output to another textfile.
Since apache uses www-data as user account, i manage to log in to terminal
using www-data, configure it to execute program.exe and it worked. But
when i run program.exe using php exec, it does not work.
I use 'whoami' to determine the user account on the job, it outputted
www-data.