How to call a function inside of another function in PHP OOP

I'm making my own custom CMS using PHP Object Oriented... I have made a page where a main admin of a site can add another admin. In order to do this I made this form:

<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">
                <option>Main Admin</option>
                <option>Administrator</option>
                <option>Content Creator</option>
                <option>Analyst</option>
            </select>
        </div>
    </div>
    <div class="box-footer">
        <button name="submit" type="submit" class="btn btn-primary">Submit</button>
    </div>
</form>

Then I added this as action file:

if (isset($_POST['submit'])) {
    $username = $_POST['uname'];
    $email = $_POST['email'];
    $password = $_POST['pass'];
    $registration = new Register();
    $registration->CheckUname($username,$email);
}

So as you can see I have called a class called Register and in this class I coded this:

class Register {
    private $db;

    public function __construct() {
        $this->db = new Connection();
        $this->db = $this->db->dbConnect();
    }

    public function CheckUname($username,$email) {
        if(!empty($username)&&($email)) {
            $chk1 = $this->db->prepare("SELECT username FROM admins WHERE user_name= ?");
            $chk1->bindParam(1,$username);
            $chk1->execute();
            if($chk1->rowCount() == 1)
            {
                $notice['username_exists'] = "Try different username";
            } else {
                $chk2 = $this->db->prepare("SELECT email 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";
                }else{
                    // I want to call the NewAdmin function here
                }
            }
        }
    }

    public function NewAdmin($username,$email,$password) {
        if(!empty($username)&&!empty($email)&&!empty($password)) {
            $reg = $this->db->prepare("INSERT INTO admins (user_name, email_address, password_hash) VALUES ( '?', '?', '?')");
            $reg->bindParam(1,$username);
            $reg->bindParam(2,$email);
            $reg->bindParam(2,$password);
            $reg->execute();
        }
    }
}

Basically I tried to check if that username already exists or not then I also checked if the email exists and if not I want to call the NewAdmin function so the admin can be inserted to db. The problem is I don't know how to do that inside of the CheckUname function. Any help please ?

Also I've got another question which is why the errors that should be produced when the user enter a username or email that already exists in database does not shown! However I have set a variable for this:

if($chk1->rowCount() == 1) {
    $notice['username_exists'] = "Try different username";
} else {
    $chk2 = $this->db->prepare("SELECT email 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";
    } else {
        // I want to call the NewAdmin function here
    }
}

I have also set the error in the page like this:

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>";
}

But no errors appears on the page while submitting a username or an email that exists in the db!