understanding a error in php membership system - php

Question

I am building a membership system in php using sessions, functions and MySQL querys,
I have come across a problem or a error that I do not understand the meaning and how to correct it.
any help on this matter would be greatly appreciated.
Error reads
Fatal error: Call to a member function error() on a non-object in /home/ob219/public_html/membership/index.php on line 6
code for index
$user = DB::getInstance()->get('users', array('username', '=', 'ben'));
if($user->error()) {
echo 'No user';
} else {
echo 'OK!';
}
db.php function
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 error() {
return $this->_error;
}

Answer 1

was a simple error, I had Users as table name, and in index.php I had users.
Solution
<?php
require_once 'core/init.php';
$user = DB::getInstance()->get('Users', array('username', '=', 'ben'));
if((!$user->count())) {
echo 'No user';
} else {
echo 'OK!';
}