CGI (in PHP) Can't Read POST Data

I am trying to have a CGI script write text to a file based on input
from a form. However my write.cgi's $_POST is empty. Whenever I use
write.php for the action of the form $_POST has the correct data.

The form uses POST method. I want to use write.cgi because only cgi
scripts can write to files in my server (run by suexec)

I have been researching this for two days now