Pdftk无法通过PHP工作

Question

I am trying to use pdftk through PHP. It works perfectly through the terminal, and some other commands work with PHP. For example:

echo 'PWD: ' . shell_exec('pwd') . '<br>';
echo 'LS: ' . shell_exec('cd processes && ls') . '<br>';
echo 'PDFTK: ' . shell_exec('/usr/bin/pdftk --help');

returns the following:

PWD: /opt/lampp/htdocs/WebApp  
LS: auto_complete_test.php .... other files I have  
PDFTK:

I cannot get any commands involving pdftk to work through PHP, and I have run out of ideas. Any help or suggestion is greatly appreciated.