login form with https and http

i want to use https only for login form and after the user logs in send it to http .

login form is in page "a.php" and the action of login form is "b.php" .

my question is:

• which one should i call: https://mysite.com/a.php or http://mysite.com/a.php

• the action should be : https://mysite.com/b.php or http://mysite.com/b.php

i mean post data is send according to form page or action page protocol??

The page with input fields (a.php) should be on HTTP, while the page with authentication logic (b.php) should be on HTTPS.

In a.php there will be something like this:

<form method="post" action="https://[domain-name]/b.php">
    <input type="text" name="username" />
    <input type="password" name="password" />
    <input type="submit" value="login" />
</form>