I have the following script to take an uploaded PDF in PHP and call ImageMagik/Ghostscript to convert to images of specified size.

Code:
$tmp = exec("convert \"{$strPDF}\" -resize 500X500 \"{$strOut}\"", $results);
However, this doesnt seem to work. No errors in the log file and no errors on screen. If I do the following,

Code:
$tmp = exec("convert \"{$strPDF}\" -resize 500X500 \"{$strOut}\"", $results); 
echo ("convert \"{$strPDF}\" -resize 500X500 \"{$strOut}\"");
And I paste the output into a command prompt it works fine (It takes around 6-10 seconds - My max_execution_time is at 600.

Any suggestions on why this might not be working?

This is Windows, IIS 7 and PHP5.

Edit: I'm having the same issue in both CentOS and Windows. Both have ImageMagik and Ghostscript installed.