[linuxthefish]

Expect allows you to "feed" the password to the passwd command.

http://linux.die.net/man/5/passwd
http://linux.die.net/man/1/expect

EDIT: Small script that might help

Код:

#!/usr/bin/expect -f
spawn passwd [lindex $argv 0]
set pass [lindex $argv 1]
expect {
        password: {send "$pass\r" ; exp_continue}
        eof exit
}

Place this script in /usr/bin and name it setpasswd. You can then call the command setpasswd username password in your PHP function or exec().