How to code correct (if check) for username that already registered

ELSE IF is not working correctly. Can u please correct my code and explain how its works. Thank you!

Here is my code php code.

<?php
    include_once('config.php');

    if(isset($_POST['username'])){
        $username = $_POST['username'];
        $first_name = $_POST['first_name'];
        $last_name = $_POST['last_name'];
        $email = $_POST['email'];
        $phone = $_POST['phone'];
        $password = $_POST['password'];

        if(strlen($password)<6){
            exit ('Password must contain 6 symbols');
        }

        $password = md5($password);
        $query ="INSERT INTO register(username,first_name,last_name,email,phone,password) VALUES ('$username', '$first_name', '$last_name', '$email', '$phone', '$password')"
        or die(mysqli_error($connect));

        if($result=mysqli_query($connect,$query)) {
            echo '<h3><font color="red">You have successfully registered</font></h3>';
        }else if (mysqli_num_rows($result)==0) {
            exit("Username already exists");
        }
    } 

?>