I'm not sure what shell you are going to get with this function, but you can find out like this:
<?php
$cmd = 'set';
echo "<pre>".shell_exec($cmd)."</pre>";
?>
On my FreeBSD 6.1 box I get this:
USER=root
LD_LIBRARY_PATH=/usr/local/lib/apache2:
HOME=/root
PS1='$ '
OPTIND=1
PS2='> '
LOGNAME=root
PPID=88057
PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin
SHELL=/bin/sh
IFS='
'
Very interesting. Note that the PATH may not be as complete as you need. I wanted to run Ghostscript via ImageMagik's "convert" and ended up having to add my path before running the command:
<?php
$cmd = 'export PATH="/usr/local/bin/"; convert -scale 25%x25% file1.pdf[0] file2.png 2>&1';
echo "<pre>".shell_exec($cmd)."</pre>";
?>
ALSO, note that shell_exec() does not grab STDERR, so use "2>&1" to redirect it to STDOUT and catch it.