Why does this code hash null?

问题

The following code passes on the username check, but fails on the password.

As you can see, the hashes are echoed, but for some reason, they output e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855, which is the sha256sum of /dev/null. As the password does not seem to echo at all, i can only assume it cannot get the POST, but why?

login

<form action="dologin" method="post">
Username: <input type="text" name="username">
Password: <input type="password" name="password">
<input type="submit">
</form>

dologin

    if ( $_POST[username] == $actualusername ) {
        // Hash the password
        $hashedpassword = hash('sha256', $_POST[password]);
        echo $_POST[password];
        echo $hashedpassword;
        if ( $hashedpassword == $actualpassword ) {
            echo '<h2>Logged in</h2>';
        } else {
        echo '<h2>Incorrect password</h2>';
        echo $hashedpassword;
        }
    } else {
        echo '<h2>Incorrect username</h2>';
    }