Validate multiple input in CI 4

Hi,

This is the code in my controller:

PHP Code:

$fields = array(
    "user_id[]" => "required"
);

print_r($this->request->getPost("user_id"));

$final_fields = array();

//to remove the fields if the post request doesn't found
foreach ($fields as $field => $validate) {
    if (!is_null($this->request->getPost($field))) {
        $final_fields[$field] = $validate;
    }
}

if (!$final_fields) {
    return true;
}

$validate = $this->validate($final_fields);

if (!$validate) {
    $validation = \Config\Services::validation();
    $message = $validation->getErrors();
    echo json_encode($message);
    exit();
} 


I'm getting this output:

Code:

Array
(
    [0] => 7
    [1] => 17
)
{"user_id[]":"The user_id[] field is required."}

Looking for help. 
Thanks in advance.