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I have made a log in form for my project. The error message is being displayed when I'm on localhost, but it does not show up when I upload it to the server, instead the page refreshes when Log in button gets clicked.
What am I doing wrong?

 

Here is the source:

//This is the logic. This passes the data to the auth.php page (see code below)
<?php
require 'database.php';
require 'constants.php';

if (isset($_POST['username']) && isset($_POST['password'])) {

	    function validate($con){
        $con = trim($con);
	    $con = stripslashes($con);
        $con = htmlspecialchars($con);
	    return $con;
	}

	$username = validate($_POST['username']);
	$pass = validate($_POST['password']);

  	//If username/password field is empty -> error message
	if (empty($username)) {
		header("Location: " . ROOT_URL . "auth.php?error=Username is needed");
	    exit();
	}else if(empty($pass)){
        header("Location: " . ROOT_URL . "auth.php?error=Password is needed");
	    exit();
	}else{

		$result = $con->query("SELECT * FROM users WHERE username='$username' AND password='$pass'");

      	//If username/password fields are correct/wrong -> move to index page/error message
		if ($result->num_rows === 1) {
			$row = $result->fetch_assoc();
            if ($row['username'] === $username && $row['password'] === $pass) {
            	$_SESSION['username'] = $row['username'];
            	header("Location: " . ROOT_URL);
		        exit();
            }else{
				header("Location: " . ROOT_URL . "auth.php?error=Wrong username or password!");
		        exit();
			}
		}else{
			header("Location: " . ROOT_URL . "auth.php?error=Wrong username or password!");
	        exit();
		}
	}
	
}else{
	header("Location: " . ROOT_URL . "auth.php");
	exit();
}

Here is frontend part with the error feedback:

<?php
//Getting the database informations
require 'config/database.php';
?>

//Getting the information from the code above shown
<form action="success-auth.php" method="POST"> <!-- <?= ROOT_URL ?> -->
    <header>
        <div class="main-header">
          //The error message, which only works in localhost
            <?php if (isset($_GET['error'])) { ?>
                <p class="alert__message error"><?php echo $_GET['error']; ?></p>
                <?php } ?>
                <div class="inp">
                    <input type="text" name="username" placeholder="Username">
                </div>
                <div class="inp">
                    <input type="password" id="btn" name="password" placeholder="Password">
                </div>
                <p><button type="submit" name="submit">Log in</button></p>
        </div>
    </header>
</form>

 I can't find what the issue is. Any kind of help is appreciated!