php oop not submitting to db - php

Question

I know a bit of php and starting to learn php oop and following a guide from alex at php academy how every when trying to insert into a db it doesn't seem to insert.
I have db detail set up like this
init.php
<?php
session_start();
$GLOBALS['config'] = array(
'mysql' => array(
'host' => '127.0.0.1',
'username' => 'root',
'password' => '',
'db' => 'oop'
),
'remember' => array(
'cookie_name' => 'hash',
'cookie_expiry' => 2592000
),
'session' => array(
'session_name' => 'user'
)
);
spl_autoload_register(function($class){
require_once 'classes/' . $class . '.php';
});
require_once 'functions/sanitize.php';
?>
this is my database class every thing seems to work fine untill i get to public function insert
db.php
<?php
class db {
private static $_instance = null;
private $_pdo,
$_query,
$_error = false,
$_result,
$_count = 0;
private function __construct(){
try {
$this->_pdo = new PDO('mysql:host=' . config::get('mysql/host') . ';dbname=' . config::get('mysql/db'), config::get('mysql/username'), config::get('mysql/password'));
} catch (PDOException $e) {
die($e->getMessage());
}
}
public static function getInstance() {
if(!isset(self::$_instance)){
self::$_instance = new db();
}
return self::$_instance;
}
public function query($sql, $params = array()) {
$this->_error = false;
if($this->_query = $this->_pdo->prepare($sql)){
$x = 1;
if(count($params)) {
foreach($params as $param){
$this->_query->bindValue($x, $param);
$x++;
}
}
if($this->_query->execute()) {
$this->_result = $this->_query->fetchAll(PDO::FETCH_OBJ);
$this->_count = $this->_query->rowCount();
} else {
$this->_error = true;
}
}
return $this;
}
public function action($action, $table, $where = array()) {
if(count($where) === 3) {
$operators =array('=', '>', '<', '>=', '<=');
$field = $where[0];
$operator = $where[1];
$vaule = $where[2];
if(in_array($operator, $operators)) {
$sql = "{$action} FROM {$table} WHERE {$field} {$operator} ?";
if (!$this->query($sql, array($vaule))->error()) {
return $this;
}
}
}
return false;
}
public function get($table, $where) {
return $this->action('SELECT *', $table, $where);
}
public function delete() {
return $this->action('DELETE', $table, $where);
}
public function insert($table, $fields = array()){
if(count($fields)){
$keys = array_keys($fields);
$values = '';
$x = 1;
foreach($fields as $field){
$values .= '?';
if($x < count($fields)){
$values .= ', ';
}
$x++;
}
$sql = "INSERT INTO users (`" . implode('`,`', $keys) . "`) VALUES ({$values})";
if(!$this->query($sql, $fields)->error()){
return true;
}
}
return false;
}
public function results(){
return $this->_result;
}
public function first() {
return $this->results()[0];
}
public function error() {
return $this->_error;
}
public function count() {
return $this->_count;
}
}
?>
and this is what I am using to submit to db
index.php
<?php
require_once 'core/init.php';
$user = db::getInstance()->insert('users', array(
'username' => 'pravaut',
'passowrd' => 'test',
'salt' => 'salt'
));
?>