Untitled

<?PHP
    include_once("connection.php");
    if(isset($_POST['txtUsername']) && isset($_POST['txtPassword']) && isset($_POST['txtAgain'])) {

        $username = $_POST['txtUsername'];
        $password = $_POST['txtPassword'];
        $again = $_POST['txtAgain'];

        $duplicates = "SELECT username FROM registracije WHERE username='$username'";
        $duplicate = mysqli_query($duplicates);

        if (mysqli_num_rows($duplicate) > 0) {
        	echo "username_exists";
        	exit();
        } else {
        	$insertRegistration = "INSERT INTO registracije (username, password)
        						 VALUES ('$username', '$password')";

	        $result = mysqli_query($conn, $insertRegistration);

	        if ($result) {
	        	echo "registration_successful";
                exit();
	        }
        }

}

?>
<html>
<head><title>Register</title></head>
    <body>
        <h1>Register</h1>
        <form action="<?PHP $_PHP_SELF ?>" method="POST">
            Username <input type="text" name="txtUsername" value="" /><br/>
            Password <input type="password" name="txtPassword" value="" /><br/>
            Password again<input type="password" name="txtAgain" value="" /><br/>
            <input type="submit" name="btnSubmit" value="Login"/>
        </form>
    </body>
</html>