PDO类-语法错误或访问冲突1064

我正在创建一个用于连接到数据库的类，但不断收到一条消息，说我的sql语法有错误，当回显我得到的查询时SELECT id, username from :table where :row = :value，它似乎没有任何错误。

<?php 
    class db{
        protected $datab;
        public function __construct($username, $password, $host, $dbname, $options){
            try { 
                $this->datab = new PDO("mysql:host={$host};dbname={$dbname};charset=utf8", $username, $password, $options); 
            } 
            catch(PDOException $ex) { 
                die("Failed to connect to the database: " . $ex->getMessage()); 
            }
            $this->datab->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); 
            $this->datab->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC); 
        }
        public function select($array, $table, $row, $value){
            $query = "SELECT";
            foreach($array as $val) {

                if ($val === end($array)){
                    $query.= " ".$val;
                    }else{
                    $query.= " ".$val.",";
                }
            }
            $query.=" FROM :table WHERE :row = :value";
            $query_params = array(
            ':table' => $table,
            ':row' => $row,
            ':value' => $value
            ); 
            echo $query; // SELECT id, username from :table where :row = :value
            try{ 
                $stmt = $this->datab->prepare($query); 
                $result = $stmt->execute($query_params); 
            } 
            catch(PDOException $ex) { 
                die("Failed to run query: " . $ex->getMessage()); 
            }
            $row = $stmt->fetch(); 
            return $row;
        }
    }
    $kit = new db("root", "", "localhost", "kit", array(PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8'));
    $kit->select(array("id", "username"), "user", "username", "yusaf");

?>  

错误信息：

Failed to run query: SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''user' where 'username' = 'yusaf'' at line 1

编辑

我很不高兴，请评论一下你为什么给我这个。我想我应该提到这只是出于学习目的，我实际上并不会在没有意义的应用程序中使用此类。