Decode hash sha256 encryption, knowing the salt

As mentioned in the comments of your question, reversing the hash is not really an option.

What you can do however, and this is what everybody else does as well. In your registration code (ex. register.php) which your form post to you can make the PHP script send the password in an email and then encrypt it and store it in the database.

I suppose you have a registration form of some kind, and that form supposedly posts the new users details to another (or the same) php script, doesn't it?

For example if my form said something like <form method="post" action="register.php">

And in register.php I would then have something like

<?php$username = mysql_real_escape_string($_POST['username']);$password = mysql_real_escape_string($_POST['password']); /*cleartext*/$email    = mysql_real_escape_string($_POST['email']);mail($email,"New account","Your username \"$username\" and your password is \"$password\"");$salt ="sometext";$escapedPW="userpass";$saltedPW =  $escapedPW . $salt;$hashedPW = hash('sha256', $saltedPW);mysql_query("INSERT INTO users (username, password, email) VALUES ($username, $hashedPW, $email)")

Some rough example code. I hope it helps!

You should NEVER send plaintext passwords via email. Rather, send a time-limited, single-use "reset password" link, as suggested in the comments.

You should not use a simple hash as suggested by @Henrik. Use a standard adjustable-work password KDF (PBKDF2,bcrypt,scrypt)

If you can use PHP 5.5, use the standard password hashing functions. There are hosts which do support PHP 5.5, but you have to look for them and ask for it.

There are many places on the web that explain how to do it correctly (e.g. https://wiki.mozilla.org/WebAppSec/Secure_Coding_Guidelines#Authentication) and many that explain how to do it incorrectly. PLEASE take some time to research this before you decide to roll your own authentication system.