php - PHP post form hidden input

Question

I have a form

<form class="well form-search" method="post">
    <input type="hidden" name="act" value="add"/>
    <input type="text" name="uname" placeholder="User name" />
    <input type="password" name="passwd" placeholder="Password" />
    <button type="submit" class="btn">Add User</button>
</form>

'uname' and 'passwd' end up in $_POST but 'act' is in $_GET for some reason.