Use the following for a guest button:
<form action="login.php" method="post">
<input name="username"type="hidden" value="guest" />
<input name="password" type="hidden" value="guest" />
<input type="submit" value="Guest Access" />
</form>
However use the following for a link:
<a href="login.php?username=guest&password=guest">Guest Access</a>