Form that logs in to a website? - php

Question

If there is a website and I want to write a form that logs me in, how can I do it?
This form logs me to stack overflow
<form method="post" action="https://stackoverflow.com/users/login">
<input type="hidden" name="email" value="myemail">
<input type="hidden" name="password" value="mypassword">
<input type="submit">
</form>
I want to do so on this website, but the script doesn't work for some reason.
<form method="post" action="http://forums.heroesofnewerth.com/login.php?do=login/">
<input type="hidden" name="vb_login_username" value="myusr">
<input type="hidden" name="vb_login_password" value="mypass">
<input type="submit">
</form>
Any idea why?
Ok, it worked I had to send more data.
<form method="post" action="http://forums.heroesofnewerth.com/login.php" onsubmit="md5hash(vb_login_password, vb_login_md5password, vb_login_md5password_utf, 0)">
<input type="hidden" name="vb_login_username" value="usr">
<input type="hidden" name="vb_login_password" value="pwd">
<input type="hidden" name="do" value="login">
<input type="hidden" name="vb_login_md5password" value="">
<input type="hidden" name="vb_login_md5password_utf" value="">
<input type="hidden" name="s" value="">
<input type="hidden" name="securitytoken" value="guest">
<input type="hidden" name="url" value="http://forums.heroesofnewerth.com/index.php">
<input type="submit">
</form>

Answer 1

A lot of websites on the internet (not nearly enough though) have protection in place that prevents sites other then their own to post forms (log in for example) to their site. A site that does not have this protection is vulnerable to:
Cross Site Request Forgery (CSRF): http://en.wikipedia.org/wiki/Cross-site_request_forgery
This is a major security risk that allows phishing sites to log you in to the actual website while catching your login details and a whole lot of other nasty stuff.
There could also be other protection in place to prevent you from sending a request.
Try to see if there are any API's available instead for what you are trying to achieve.