SQL注入安全吗？

Question

我创建了一个连接到我的数据库并插入一些值的类。它是安全的还是我如何进一步保护注射？在验证之后，对象声明将来自表单中POST的变量ofc。只是想知道这是否安全。

<?php
include "db/db_info.php";
/*$DBServer
$DBUser
$DBPass
$DBName*/
class WorkDB {
    private $server;
    private $user;
    private $pass;
    private $name;
    private $conn;

    public function __construct( $server, $user, $pass, $name ) {
        $this->server=$server;
        $this->user=$user;
        $this->pass=$pass;
        $this->name=$name;
    }

    public function tryconn() {
        $this->conn = new mysqli(  $this->server, $this->user, $this->pass, $this->name );

    if ( $this->conn->connect_error ) {
        die( 'Connection Error (' . $this->connconnect_errno . ') '
                . $this->conn->connect_error );
    }


    else echo 'ok';

}

public function query_register( $user, $pass, $email ) {
    $stmt = $this->conn->prepare( "INSERT INTO `users` (`username`, `password`, `email`) VALUES (?, ?,?)" );
    $stmt->bind_param( "sss", $user, $pass, $email );
    $stmt->execute();
    $stmt->close();
}


}//end of class


$a=new WorkDB( $DBServer, $DBUser, $DBPass, $DBName );
$a->tryconn();
$a->query_register( 'a', 'b', 'c' );


?>