Can't get exec() to run pdflatex

[jbrukh]

I am trying to get PHP to compile a LaTeX file for a web user.

I basically need to run

exec("pdflatex myvalidfile.tex", $err);

This works fine on my server command line and all www permissions are set correctly. However, every time the php file is run by www, $err is set to 127.

Any ideas?

Jake.