External PHP login script

[Becca]

Hello!

I am developing some software for users to use with the forum. Activity in the forum and in the software will affect each other, so the user must login to the software using their forum account.

I am curious as to how i could use the users existing password to achieve this, instead of creating a new password for each user. I understand the basics of PHP but do not understand the more complex elements such as how the passwords are encrypted for Vbulletin.

Does anyone know if there is a simple example PHP script out there which does the following:

$username = the inputted username
$password = the inputted password

if $password = password in the Vbulletin database for $username
  echo "login success"
else
  echo "login failed"