How to through error message in Yii2 Api? - api

Question

Here is the code attached.
How can I throw an error message if any attribute is missing?
I need to check if all attributes are coming from the form or not. If any attribute is missing I want to throw an error message with proper error code.
$get_email = Null;
$get_password = Null;
$get_username = Null;
if (isset($_POST['username']) ) {
$get_username = $_POST['username'];
}
if (isset($_POST['email']) && isset($_POST['password']) ) {
$get_email = $_POST['email'];
$get_password = $_POST['password'];
}
if ($get_email == Null || $get_username == Null || $get_password == Null ) {
// through error code with message
}
else {
$model = new SignupForm();
$model->email = $get_email;
$model->username = $get_username;
$model->password = $get_password;
}

Answer 1

Here is the code through which I have achieved my goal.
$model = new SignupForm;
if (isset($_POST['username'] )) {
$model->username = $_POST['username'];
}
if (isset($_POST['email'] )) {
$model->email = $_POST['email'];
}
if (isset($_POST['password'] )) {
$model->password = $_POST['password'];
}
if($model->validate()){
$model->signup();
}
else{
return $model->getErrors();
}