Need to reorder my code to prevent reject email address and duplicate in username field?

everything on my form is working i need to reorder the if, else if and else statement to work likes this:

if user name = already taken username reject adding data to database and show message

else if = username contains an @ reject adding data to database and show message

else = submits data to database, send user to verification page and send a email with a verification code:

if you could leave a comment with the readjusted code would be much appreciated as i need it to be done for Thursday:

<?php
session_start();

// connect to database
$db = mysqli_connect("localhost", "root", "", "authentication")
or die('Sorry the Database is not Available<br />');

$username = "";
$password = "";
$email = "@";
$query = mysqli_query ($db, "SELECT * FROM customers WHERE username='$username'");

if (isset($_POST['register_btn']))
    {
        mysqli_num_rows($query) > 0;
        echo "Username is already in use!"; 
    }
    else if( ! filter_var($email,FILTER_VALIDATE_EMAIL ) )
    { // "FILTER_VAR" RETURNS THE EMAIL (STRING) IF IT WAS VALID, failed!
        echo "Username rejected (don't enter your email as username)";
    }
    else {
    $username = mysqli_real_escape_string($_POST['username']);
    $password = mysqli_real_escape_string($_POST['password']);
    $email = mysqli_real_escape_string($_POST['email']);    
    // creates username
    $password = password_hash($password); //hash password before storing for security purposes
    $sql = "INSERT INTO customers(username, password, email) VALUES('$username', '$password', '$email')";
    mysqli_query($db, $sql);
    echo("location: verification.php"); //redirect to home page
    }
?>