Unexpected behaviour with passthru in php

Question

I've set up nginx and php on an ubuntu server 12.04. I have the following php file:

<?php
echo "Hello ";
echo passthru("cat /proc/meminfo");
echo "world. I am ";
echo passthru("whoami");   
?>

This works completely fine when run from the command line, and php returns the text from /proc/meminfo. When run via nginx, however, I see this:

Hello world. I am www-data 

Clearly, passthru is working. However, I can't seem to read anything in /proc. The permissions on /proc is dr-xr-xr-x, and I have confirmed that the www-data user that nginx and php run as can access that directory. I've turned on the php logging, and there are no error messages related to this.

Suggestions?

Edit: Safe mode is turned off.

Answer 1

So, we fixed this offline -- I'm putting this up here for the next person who has this issue:

We verified that using the full path to cat resolved the problem. There was no visible error because passthru doesn't capture stderr (and the error was a failure to find cat). The River then discovered that $PATH was being set to the wrong thing, and fixed it.