$_GET is for when the variable is in the URL.
$_POST is for when the variable is posted.
Had you done
PHP Code:
<form action="<?php echo ($_SERVER['PHP_SELF']); ?>" method="POST">
<input type="hidden" name="p" value="submitted" />
Admin Username: <input type="text" name="adminuser" accesskey="u" /><br />
Admin Password: <input type="text" name="adminpass" accesskey="p" />
</form>
You would have had to use $_POST
Note also that had you done
PHP Code:
<form action="<?php echo ($_SERVER['PHP_SELF']); ?>" method="GET">
<input type="hidden" name="p" value="submitted" />
Admin Username: <input type="text" name="adminuser" accesskey="u" /><br />
Admin Password: <input type="text" name="adminpass" accesskey="p" />
</form>
That all variables would be in the URL when the form was submitted and subsequently they'd all be available via $_GET (not too bright for a username/password, but I am trying to give an example)
BTW: It doesn't hurt to give people a submit button.
Few will realize that they can just hit enter.