PHP OOP Does Not Show Error Messages On Form Submitting

I'm working on a project using PHP OOP. Basically for this project I've made a page for adding a new Admin for the site.

So I coded this:

<?php 
if (isset($_POST['submit'])){
    $username = $_POST['uname'];
    $email = $_POST['email'];
    $password = $_POST['pass'];
    $groups = $_POST['groups'];
    if($groups == "Administrator"){
        $level = 3;
    }else if($groups == "ContentCreatorBlog"){
        $level = 4;
    }else if($groups == "ContentCreatorShop"){
        $level = 5;
    }else if($groups == "ContentCreatorGallery"){
        $level = 6;
    }else if($groups == "Secretary"){
        $level = 7;
    }else if($groups == "SocialMediaManager"){
        $level = 8;
    }else if($groups == "Analyst"){
        $level = 9;
    }else{
        $level = Null;
    }
    if (filter_var($email, FILTER_VALIDATE_EMAIL) === false) {
        $notice['email_validation'] = "The email that you have entered is not a valid one";
    }else{
        $registration = new Register();
        $notice[] = $registration->CheckUname($username,$email,$password,$groups,$level);   
    }   
}
?>
<section class="content">
    <div class="row">
        <div class="col-md-1">
        </div>
        <div class="col-md-10">
            <div class="box box-primary" id="myModal1">
                </br>
                <div class="box-header with-border">
                    <h3 class="box-title">Required Information</h3>
                </div>
                <?php 
                if(isset($notice['email_validation'])) {
                    echo "
                        <div class='alert alert-danger'>
                            <strong>Hey!</strong> ".$notice['email_validation'].".
                        </div>
                    ";
                }
                if(isset($notice['username_exists'])) {
                    echo "
                        <div class='alert alert-danger'>
                            <strong>Hey!</strong> ".$notice['username_exists'].".
                        </div>
                    ";
                }
                if(isset($notice['email_exists'])) {
                    echo "
                        <div class='alert alert-danger'>
                            <strong>Hey!</strong> ".$notice['email_exists'].".
                        </div>
                        ";
                }
                if(isset($notice['success_message'])) {
                    echo "
                        <div class='alert alert-success'>
                            <strong>Hey!</strong> ".$notice['success_message'].".
                        </div>
                ";
                }
                ?>
                <form role="form" method="POST" action="">
                    <div class="box-body">
                        <div class="form-group">
                            <label>User name:</label>
                            <input type="text" class="form-control" placeholder="Enter username" name="uname" required>
                        </div>
                        <div class="form-group">
                            <label for="exampleInputEmail1">Email address:</label>
                            <input type="email" class="form-control" id="exampleInputEmail1" placeholder="Enter email" name="email" required>
                        </div>
                        <div class="form-group">
                            <label for="exampleInputPassword1">Temporary password:</label>
                            <input type="password" class="form-control" id="exampleInputPassword1" placeholder="Enter password" name="pass" required>
                        </div>
                        <div class="form-group">
                            <label>Group admin:</label>
                            <select class="form-control" name="groups">
                                <option value="Administrator">Administrator</option>
                                <option value="ContentCreatorBlog">Blog Content Creator</option>
                                <option value="ContentCreatorShop">Shop Content Creator</option>
                                <option value="ContentCreatorGallery">Gallery Content Creator</option>
                                <option value="Secretary">Secretary</option>
                                <option value="SocialMediaManager">Social Media Manager</option>
                                <option value="Analyst">Analyst</option>
                            </select>
                        </div>
                    </div>
                    <div class="box-footer">
                        <button name="submit" type="submit" class="btn btn-primary">Submit</button>
                    </div>
                </form>
                </br>
            </div>
        </div>
        <div class="col-md-1">
        </div>
    </div>
</section>

So what it simply does is to grab some Variables such as $username, $email,passowrd & $groups from what user has submitted and call a Class named Register which goes like this:

<?php 
class Register
{   
    protected $notice = array();
    private $_db;
    public function __construct()
    {
        $this->_db = new Connection();
        $this->_db = $this->_db->dbConnect();
    }
    public function CheckUname($username,$email,$password,$groups,$level)
    {
        if(!empty($username)&&!empty($email)&&!empty($password)&&!empty($groups)&&!empty($level))
        {
            $chk1 = $this->_db->prepare("SELECT user_name FROM admins WHERE user_name = ?");
            $chk1->bindParam(1,$username);
            $chk1->execute();
            if($chk1->rowCount() == 1)
            {
                $notice['username_exists'] = "Try different username";
                return $this->notice;
            }else{
                $chk2 = $this->_db->prepare("SELECT email_address FROM admins WHERE email_address = ?");
                $chk2->bindParam(1,$email);
                $chk2->execute();
                if($chk2->rowCount() == 1)
                {
                    $notice['email_exists'] = "The email address that you have entered is already exists in database";
                    return $this->notice;
                }else{
                    $this->NewAdmin($username,$email,$password,$groups,$level);
                    $notice['success_message'] = "New admin was successfully added";
                    return $this->notice;
                }
            }
        }
    }
    public function NewAdmin($username,$email,$password,$groups,$level)
    {
        if(!empty($username)&&!empty($email)&&!empty($password)&&!empty($groups)&&!empty($level))
        {
            $reg = $this->_db->prepare("INSERT INTO admins (user_name, email_address, password_hash, group_admin, date_joined, admin_level) VALUES ( ?, ?, ?, ?, NOW(), ?)");
            $reg->bindParam(1,$username);
            $reg->bindParam(2,$email);
            $reg->bindParam(3,$password);
            $reg->bindParam(4,$groups);
            $reg->bindParam(5,$level);
            $reg->execute();
        }
    }
    public function getNotice()
    {
        return $this->notice; 
    }
}
?>

And if any kinds of messages occurs (including error messages and success message which I have determined on the Class), such as "Try different username" or ""The email address that you have entered is already exists in database"" or "New admin was successfully added";, should be appeared above the form just like this:

if(isset($notice['email_validation'])) {
    echo "
        <div class='alert alert-danger'>
            <strong>Hey!</strong> ".$notice['email_validation'].".
        </div>
    ";
}
if(isset($notice['username_exists'])) {
    echo "
        <div class='alert alert-danger'>
            <strong>Hey!</strong> ".$notice['username_exists'].".
        </div>
    ";
}
if(isset($notice['email_exists'])) {
    echo "
        <div class='alert alert-danger'>
            <strong>Hey!</strong> ".$notice['email_exists'].".
        </div>
    ";
}
if(isset($notice['success_message'])) {
    echo "
        <div class='alert alert-success'>
            <strong>Hey!</strong> ".$notice['success_message'].".
        </div>
    ";
}

But now the problem is, popping up message does not work out at all! I mean if you enter an repeated E-mail address which exists at db, the $notice['email_exists'] message does not show up.

And if you fills the form successfully, the $notice['success_message'] message does not appear however, a new record inserts into db.

So how to fix this problem... I have done everything that I could do, so please if you know how to solve my question, please let me know.. I would really really appreciate that!

Thanks in advance