PHP OOP issue with database

I am querying usernames from database using the get() function in the DB class.. its always returning 'No users' even if there are users existing in the database.. here is my DB.php

<?php
class DB{
    private static $_instance = null;
    private $_pdo, 
            $_query, 
            $_error = false, 
            $_results, 
            $_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, $params);
                    $x++;
                }
            }

            if ($this->_query->execute()) {
                $this->_results = $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];
            $value    = $where[2];

            if (in_array($operator, $operators)) {
                $sql = "{$action} FROM {$table} WHERE {$field} {$operator} ?";

                if(!$this->query($sql, array($value))->error()){
                    return $this;
                }
            }
        }
        return false;
    }

    public function get($table, $where) {
        return $this->action('SELECT *', $table, $where);
    }

    public function delete($table, $where) {
        return $this->action('DELETE', $table, $where);
    }

    public function results(){
        return $this->_results;
    }

    public function error(){
        return $this->_error;   
    }

    public function count(){
        return $this->_count;   
    }
}

index.php

<?php
require_once 'core/init.php';

$user = DB::getInstance();
$user->get('users', array('username', '=', 'alex'));

if(!$user->count()) {
    echo 'No user';
} else {
    foreach ($user->results() as $user) {
        echo $user->username, '<br>';
    }
}

Am getting 'No user' even if there is existing user in the the DB if I use this..

$user = DB::getInstance()->get('users', array('username', '=', 'alex'));

but it is returning correct usernames(including 'alex') if I replace get() with query()

$user = DB::getInstance()->query("SELECT * FROM users");