Help with exec() or system()

This may be more a Linux/Unix question than a PHP one. I'm not sure. In a telnet session, I can execute: /usr/local/bin/pgpe -r <username> filename.txt -o filename.pgp to create a PGP-encrypted file. In a PHP script I can do: $foo = system("/usr/local/bin/pgpe -h"); and when I echo $foo I get a display of the PGP help text. But when I tryo to execute: $foo = system("/usr/local/bin/pgpe -r <username> filename.txt -o filename.pgp"); $foo contains nothing. What am I doing wrong?