shell_exec - how to deal with prompts?

[Nate]

When i'm in SSH i can zip and encrypt a file by doing the following:

[root@www tmp]# zip -e file.zip file.txt
Enter password: *****
Verify password: *****
  adding: file.txt (deflated 71%)

My question is, how can i do this from a php script?

I tried this:

$cmd .= "zip -e test.zip test.txt;";
$cmd .= "example;example";
print_r(shell_exec($cmd . " 2>&1"));

but it errors saying: sh: line 1: example: command not found

[Nate]

Also, running the following does work from the web (same thing but doesnt encrypt it):

$cmd .= "zip test.zip test.txt;";
print_r(shell_exec($cmd . " 2>&1"));