gpg on cron task

[enediel gonzalez]

Hello:
  I use debian sarge and I need to encrypt and decrypt automatically backup 
files using gpg

  I create a php script will the whole procedure, which include the 
following declaration

<?php
  .......


  $str_execute = "cd /tmp/backup && /bin/echo apassword | /usr/bin/gpg -se 
--passphrase-fd 0 -r enediel at hotmail.com /
tmp/backup/$filename";
  exec($str_execute);

?>

enediel at hotmail.com is included into the trusted chain for the root user.

When I execute the script manually as root, I obtain the encrypted file, but 
if I put the same script as a cron's task, the whole script works perfectly 
except the encryption process, and I not receive any error.

I'll appreciate if somebody help me with that, basically what I need is to 
encrypt and decrypt specific files automatically using gpg.

Thanks in advance for any answer
Enediel
Linux user 300141
Debian GNU/Linux