致命错误:在非对象上调用成员函数prepare()，调用prepare语句的对象不为空

我已经在网上搜索了，但我没有找到任何解决我的问题的方法。我有一个类DB_Functions，使用准备语句来执行SQL查询。DB_Functions.php

<?php
 class DB_Functions {
 private $con;
 //constructor
 function __construct(){
   require_once __DIR__  . "/db_connect.php";
   //connecting to database
   $db = new DB_CONNECT();
   $this->con = $db->connect();
  / /var_dump($conn);
 }
 //destructor
 function __destruct(){
  }
//store new firm //testing
public function storeFirm($name, $email, $tel, $address, $city, $code){
    var_dump($this->con);
    $stmt = $this->con->prepare("INSERT INTO Charset (charset) VALUES (?)");
    $stmt->bind_param("s", $name);
    $stmt->execute();
    $stmt->close();
 }

 }
 ?>

变量$this->con使用var_dump返回类型为(mysql link)的资源(4)

db_connect.php

  class DB_CONNECT{
 function __construct(){
    //import database connection variables
    require_once __DIR__ . "/db_config.php";
  }
 /*
 function to connect with database
 */
function connect(){
    //connecting to mysql database
    $conn = mysql_connect(DB_SERVER, DB_USER, DB_PASSWORD) or die (mysql_error() . PHP_EOL);
    //select database
    $db = mysql_select_db(DB_DATABASE, $conn) or die (mysql_error() . PHP_EOL);
    /*check connection*/
    if ( mysql_error() ){
      printf("Connect failed: %s'n", mysql_error());
      exit();
    }
    else {
      mysql_set_charset("utf8", $conn) or die (mysql_error() . PHP_EOL);
      //$this->setCharacterSet($con);
      //returning connection cursor
      //var_dump($conn);
      return $conn;
    }
}
/*function to close db connection*/
function close(){
  //closing db connection
  mysql_close();
}
 //set utf-8 character set
 function setCharacterSet($con){
    if ( mysql_client_encoding($con) != "utf8"){
        //mysql_set_charset("utf8");
        printf("The current character set is : " . mysql_client_encoding($con) . "'n");
    }
 }
  }
?>

谁能告诉我罪魁祸首在我的代码中位于哪里?