Informations are not sent by form - php

Question

I wrote a code and i used Boostrap to make 2 dropdown : Sign In and Sign Up. After i complete input fields , informations aren't send ... and submit button redirect me to index.php?username="What is in username field"&password="Password typed" .
I wanna send data to informations index.php?tab=SignIn... What could be wrong ?
<ul class="nav navbar-nav nav pull-right">
<li class="animated dropdown">
<a href="#" class="dropdown-toggle" data-toggle="dropdown">
<i class="fa fa-lock fa-2x"></i><br /> Register <b class="caret"></b>
</a>
<ul class="dropdown-menu">
<?php
#$_SESSION['username2'] = $_POST['username'];
#$_SESSION['email'] = $_POST['email'];
#$_SESSION['password'] = $_POST['password'];
?>
<form class="form" id="formLogin" action="index.php?tab=SignUp">
<li><div class="form-group">
<input type="text" name="username" id="username" class="form-control" placeholder="Username" value="<?= $_SESSION['username2'] ?>" required/>
</div></li>
<li><div class="form-group">
<input type="email" name="email" id="email" class="form-control" placeholder="E-Mail Address" value="<?= $_SESSION['email'] ?>" required/>
</div></li>
<li><div class="form-group">
<input type="password" name="password" id="password" class="form-control" placeholder="Password" value="<?= $_SESSION['password'] ?>" required/>
</div></li>
<li class="divider"></li>
<input type="submit" name="register" class="btn btn-success" value="Register" tabindex="7">
</form>
</ul>
</li>
</ul>

Answer 1

replace this
<form class="form" id="formLogin" action="index.php?tab=SignUp">
with this
<form class="form" id="formLogin" action="index.php" method="post">
<input type="hidden" name="tab" value="SingUp" />
and in your index.php get tab parameter by $_POST['tab']

Answer 2

change
<form class="form" id="formLogin" action="index.php?tab=SignUp">
to
<form class="form" id="formLogin" action="index.php?tab=SignUp" method="post">

Answer 3

Nope. Just call index.php each time. No suffixes to the url ever. If you want a special value, then create a hidden element in the form, with the value set so the second time the user hits index.html, the value will populate...
<?php
#$_SESSION['username2'] = $_POST['username'];
#$_SESSION['tab'] = $_POST['tab']; // set in the original form in a hidden field.
#$_SESSION['email'] = $_POST['email'];
#$_SESSION['password'] = $_POST['password'];
?>
Somewhere along the line do a test if ($_POST['tab'] == "SignUp") then do whatever. If $_POST['tab'] isnt = SignUp, then you know its the first time to that page....
Keep it very simple. Less is more! (oops.. I'm a duplicate of the posting by Gabriele Pieretti) I'm upvoting GP! thx.