PDF thumbnails can't be created via exec - php

Question

I'm trying to have my web app create thumbnails for uploaded PDFs with the following command:
exec( "/usr/local/bin/convert '/path/to/file.pdf'[0] -background white -alpha remove -thumbnail 600x '/path/to/file.pdf.png'", $output, $return );
When I run this command from the command line it works, both when I run it as root and when I run it as _www (the user of the web server) using:
sudo -u _www /usr/local/bin/convert '/path/to/file.pdf'[0] -background white -alpha remove -thumbnail 600x '/path/to/file.pdf.png'
But when the exec command runs, the thumbnail is not created. The $output array is empty, and the $return value is 1 (general error).
Furthermore, when I generate thumbnails for other kinds of files, I have no problems at all. For example for GIFs I use:
exec( "/usr/local/bin/convert '/path/to/file.gif'[0] -thumbnail 600x '/path/to/file.gif'", $output, $return );
I have no problems with JPGs, PNGs or BMPs either. This leads me to think that the problem may be related to GhostScript, as only PDFs require GhostScript to generate thumbnails. As a last comment, the thumbnail generation fails both in my localhost and in my server.
I'm out of ideas, anyone has one? Thanks!

Answer 1

In the end, I had two different problems, one in my localhost, another in my server.
In my localhost, GhostScript was not in the PATH of the exec environment. To solve this, I added the following line just before the command:
putenv( 'PATH=' . getenv( 'PATH' ) . ':' . dirname( '/usr/local/bin/gs' ) );
This fixed the problem in my localhost. But when I pushed the new code to the server, it still didn't work. So I checked the PATH in my server and GhostScript was already in there. I then figured out that my server was using an old version of GhostScript, with no "alpha" support. So I simplified the command to:
exec( "/usr/local/bin/convert '/path/to/file.pdf'[0] -thumbnail 600x '/path/to/file.pdf.png'", $output, $return );
And now both my localhost and server work, yay!