将我的PHP代码连接到我的数据库时出错

Question

我收到以下错误消息：致命错误：未捕获错误：在C：\ xampp \ htdocs \ mine \ server.php中调用未定义函数mysql_real_escape_string（）：12堆栈跟踪：＃0 C：\ xampp \ htdocs \ mine \ _register.php（1）：在第12行的C：\ xampp \ htdocs \ mine \ server.php中抛出include（）＃1 {main}

这是我的代码：

<?php
     $username = "";
     $email = "";
     $errors = array();

    //connect to the database
    $db = mysqli_connect('localhost', 'root', '', 'registration');

    //if the register button is clicked
    if (isset($_POST['register'])) {

        $username = mysql_real_escape_string($_POST['username']);
        $email = mysql_real_escape_string($_POST['email']);
        $password_1 = mysql_real_escape_string($_POST['password_1']);
        $password_2 = mysql_real_escape_string($_POST['password_2']);

        //to ensure that the form fields are filled properly
        if (empty($username)) {
            array_push($errors, "Username is required");
          }

          if (empty($email)) {
            array_push($errors, "Email is required");
          }

          if (empty($password_1)) {
            array_push($errors, "Password is required");
          }

          if($password_1 != $password_2){
            array_push($errors, "The two passwords do not match");
          }

          //if there's no errors, save the user to database
          if(count($errors)==0) {
            $password = md5($password_1); //encrypt password before storing to database (security)
            $sql = "INSERT INTO users (username, email, password)
                         VALUES ('$username', '$email', 'password')";
             mysqli_query($db,$sql);
          }

    }


  ?>