Server :: Php Code To Change Password

I have the following php code to print the output in the browser

1) <?php
$one=exec("pwd");
echo $one; //output : /root/docs
$two=exec("ls -l");
echo $two; //output :drwxr-xr-x 9 root root 4096 Apr 3 04:40 .
$three=exec("smbpasswd -a bob");
echo $three; //output : prints some garbage values ..
?>

but when i execute the above code , $three is not working ... I am in a situation to change a password for samba users from php code.