<?php

 public function getAll($sort, $start, $limit)
    {
        $sql = "SELECT 
                name, second_name, group_number, summary
                FROM 
                students
                ORDER BY :sort LIMIT :start, :limit";
        $sth = $this->dbh->prepare($sql);
        $sth->bindValue(":sort", $sort);
        $sth->bindValue(":start", $start);
        $sth->bindValue(":limit", $limit);
        $sth->execute();
        return $sth->fetchAll(\PDO::FETCH_CLASS, 'models\Student');
    }