在PHP中扩展GROUP BY查询到现有的PDO get查询

这是我的问题，我遵循youtube上使用PDO作为查询的教程，更像是面向对象的php查询。我试着扩展get()函数的功能。其中主要的get()和action是对仅能得到参数WHERE的函数的扩展。下面是代码:

public function action($action, $table, $where = array()) {
    if (count($where) === 3) { // make sure 3 array
        $operators = array('=', '>', '<', '>=', '<='); // available operators
        $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);
}

我试图在这个函数中扩展GROUP BY查询。这是我到目前为止的尝试。但是我在 count()

Fatal error: Call to a member function count() on a non-object在此函数

// Find user and return first found
public function find($user = null) {
    if ($user) {
        $field = (is_numeric($user)) ? 'user_id' : 'username';
        $data = $this->_db->get('user', array($field, '=', $user));
        **if ($data->count()) {**
            $this->_data = $data->first();
            return true;
        }
    }
    return false;
}

我正在执行的查询:

DB::getInstance()->get('table', array('date', '=', $date_sort), array('car_id'));

到目前为止我所做的是:

public function action($action, $table, $where = array(), $group = array()) {
    if (count($where) === 3) { // make sure 3 array
        $operators = array('=', '>', '<', '>=', '<='); // available operators
        $field      = $where[0];
        $operator   = $where[1];
        $value      = $where[2];
        $g_field    = $group[0];
        if(in_array($operator, $operators)) {
            $sql = "{$action} FROM {$table} WHERE {$field} {$operator} ? GROUP BY {$g_field}";
            if (!$this->query($sql, array($value))->error()) {
                return $this;
            }
        }
    }
    return false;
}
//$user = DB::getInstance()->get('user', array('username', '=', 'sanbrons'));
public function get($table, $where, $group = null) {
    return $this->action('SELECT *', $table, $where, $group);
}