pdo_exception errors.. expected to be string, array given

hello I have a pdo warning, with expected to be string, array given... here is my code. am using sublime text 3 and php 7.0 and OOP approach of coding

this is db.php

<?php
class Database{
    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 Database();
        }

        return self::$_instance;
    }
}

?>