Nested if / else statements / extreme confusion

as you can see I'm trying to create a registration page and I have completely mucked up all my if statements and else statements, I'm completely lost and am just chucking in random brackets I'm that lost I was wondering if anybody could point me in the direction of sorting this out.

<?php
require_once './include/config.php';
require_once './include/functions/cleanString.php';
if ($_POST ['submit']) {
$username = $_POST['username'];
$password = $_POST['password'];
$email = $_POST['email'];
$trn_date = date("Y-m-d H:i:s");

$error_check = array();


if (!empty($username)) {
    $username = cleanString($username);
    $query ="SELECT * FROM users WHERE Username='$username'";
    $result = mysqli_query($con, $query) or exit ("Error in query: $query. ");
    if(mysqli_num_rows($result) > 0) { //check if there is already an entry for that username
            $error_check['username'] = 'Username already in use';
    }
    if(ctype_alpha($username)) {
    }else {
        $error_check ['username'] = 'Name must be alphabetic characters only';
    }
} else { 
    $error_check ['username'] = 'Please enter a username';
    }
    if (!empty($email)) {
        $email = cleanString($email);
        $query ="SELECT * FROM users WHERE email='$email'";
    $result = mysqli_query($con, $query) or exit ("Error in query: $query. ");
    if(mysqli_num_rows($result) > 0) { //check if there is already an entry for that username
            $error_check['email'] = 'EmaIl already in use';
        if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        $error_check ['email'] = 'Please enter a valid email adress';
        }
    } else {
    $error_check ['email'] = 'Please enter a email adress';
    }
     if (!empty($password)) {
        $password = cleanString($password);
     if (preg_match("/^.*(?=.{8,})(?=.*[0-9])(?=.*[a-z])(?=.*[A-Z]).*$/", $_POST["password"]) === 0) {
     $error_check ['password'] = "The password must contain lower case characters, upper case characters and numbers. It's length should be between 8 and 32 characters.";
}

}
         } else {
                 $error_check ['passsword'] = 'Please enter a password';

     }



    if (empty($error_check)) {

        $_SESSION['username'] = $username;
        $_SESSION['email'] = $email;
        $_SESSION['password'] = $password;

        $_SESSION['message'] = 'Registration Sucessful';

        header("Location: ./reg2.php");
    } else {
        $_SESSION['username'] = $username;
        $_SESSION['email'] = $email;
        $_SESSION['password'] = $password;
        $_SESSION['errors'] = $error_check;

    $query = mysqli_query($con, "INSERT into `users` (username, password, email, registerdate) VALUES ('$username', '".md5($password)."', '$email', '$trn_date')");

        $_SESSION['message'] = 'There were errors in your registration, please correct the errors and try again!';
                header("Location: ./reg2.php");
    }
}

?>