It looks like you just want to pass the password value to another page, right? I am also assuming that the page you are passing it to is under your control scope (you are designing it). If not, then this is a hacking question and I hate you.
If so, there are multiple ways to do this. I typically like to use session variables for this because they are not passed in the URL, do not require any file reads/writes, are easy to implement, and are extremely effective.
In essence, you will start the page by adding:
this will allow you to use session variable scope.
After a user enters in their form information (and the form does a post operation), you can inspect the value and put it into a session variable.
OR, have the form redirect to another page and have that page inspect the $_POST value being passed to it.