Submitting form without changing tab panes using only PHP - php

Question

I'm using Bootstrap 4 and I've created a two pane form; registration and sign in forms.
I know using AJAX is more suitable for such situations that it won't make the page refresh, however, I'm asking if there is any way of achieving that the page refreshes and previews the same pane using only PHP.
Using PHP in the code. Is there any way that when I submit the form when in registration pane (the one on the right hand side), the page refreshes and stays in the same pane.
<!-- Tabs -->
<nav class="register-nav">
<div class="nav nav-tabs nav-justified" id="nav-tab" role="tablist">
<a class="nav-item nav-link active" id="nav-signin-tab" data-toggle="tab" href="#nav-signin" role="tab">Sign in</a>
<a class="nav-item nav-link" id="nav-signup-tab" data-toggle="tab" href="#nav-signup" role="tab">Create New Account</a>
</div>
</nav>
<!-- Content of Tabs -->
<div class="tab-content" id="nav-tabContent">
<!-- Sign in Tab Pane -->
<div class="tab-pane fade show active" id="nav-signin" role="tabpanel">
<form action="" method="POST">
<div class="form-group">
<div class="row">
<div class="col-sm-6">
<a class="btn btn-block btn-social btn-facebook">
<span class="fa fa-facebook"></span> Sign in with Facebook
</a>
</div>
<div class="col-sm-6">
<a class="btn btn-block btn-social btn-google">
<span class="fa fa-google"></span> Sign in with Google
</a>
</div>
</div>
</div>
<div class="form-group">
<input type="text" class="form-control" placeholder="Username / Email Address">
</div>
<div class="form-group">
<input type="password" class="form-control" placeholder="Password">
</div>
<div class="form-group text-center mt-5">
<input type="submit" class="btn btn-outline-info" value="Sign in">
</div>
</form>
</div>
<!-- Sign Up Tab Pane -->
<div class="tab-pane fade" id="nav-signup" role="tabpanel">
<form action="" method="POST">
<div class="form-group">
<input type="text" class="form-control" placeholder="First name" required>
</div>
<div class="form-group">
<input type="text" class="form-control" placeholder="Last name" required>
</div>
<div class="form-group">
<input type="text" class="form-control" placeholder="Username" required>
</div>
<div class="form-group">
<input type="email" class="form-control" placeholder="Email Address" required>
</div>
<div class="form-group">
<input type="password" class="form-control" placeholder="Password" required>
</div>
<div class="form-group">
<input type="password" class="form-control" placeholder="Confirm Password" required>
</div>
<div class="form-check">
<input type="checkbox" class="form-check-input" required>
<label class="form-check-label">By registering to
velocity, you agree to its
terms and conditions
</label>
</div>
<div class="form-group text-center mt-5">
<input id="register-btn" type="submit" class="btn btn-outline-success" value="Register" disabled>
</div>
</form>
</div>
</div>

Answer 1

It is really not a PHP thing, but you could use PHP to manipulate the HTML to move the active class to the other tab.
<?php
$signinTabIsActive = "active";
$signupTabIsActive = "";
if(isset($_POST['firstname'])){
$signinTabIsActive = "";
$signupTabIsActive = "active";
?>
<!-- Tabs -->
<nav class="register-nav">
<div class="nav nav-tabs nav-justified" id="nav-tab" role="tablist">
<a class="nav-item nav-link <?= $signinTabIsActive; ?>" id="nav-signin-tab" data-toggle="tab" href="#nav-signin" role="tab">Sign in</a>
<a class="nav-item nav-link <?= $signupTabIsActive; ?>" id="nav-signup-tab" data-toggle="tab" href="#nav-signup" role="tab">Create New Account</a>
</div>
</nav>

Answer 2

Add a hidden input with a value that indicaties which pane should open. When generating the form (with Php) decide which pane gets an active class based on this value in the $_POST array