php - error message "mysql_connect(): The mysql extension is deprecated and will be removed in the future on line 6." how can i fix it?

here is my code. how can i fix it?
the error message goes like this

"Deprecated: mysql_connect(): The mysql extension is deprecated and
will be removed in the future: use mysqli or PDO instead in
C:\wamp\www\test12345\db.php on line 6"

how can i make a login page without errors?
im trying to make a website that has a secure login with session?. am i doing it right?

db.php

$mysql_hostname = "localhost";
$mysql_user = "root";
$mysql_password = "";
$mysql_database = "web";
$bd = mysql_connect($mysql_hostname, $mysql_user, $mysql_password) or die("Can't connect to database");
mysql_select_db($mysql_database, $bd) or die("Opps some thing went wrong");

login.php

include("db.php");
session_start();
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    // username and password sent from Form
    $username = mysql_real_escape_string($_POST['username']);
    $password = mysql_real_escape_string($_POST['password']);

    $password = md5($password); // Encrypted Password
    $sql = "SELECT id FROM admin WHERE username='$username' and passcode='$password'";
    $result = mysql_query($sql);
    $count = mysql_num_rows($result);

    // If result matched $username and $password, table row must be 1 row
    if ($count == 1) {
        session_register("username");
        $_SESSION['login_user'] = $username;


        header("location: welcome.php");
    } else {
        $error = "Your Login Name or Password is invalid";
    }
}
?>

    UserName :
    
    Password :

    
    

Answer

I haven't tested this but if you wanted an example of how to convert this to PDO

    //really basic connection wrapper class
    class DB{

        protected $_conn;

        public function __construct(){
            $mysql_hostname = "localhost";
            $mysql_user = "root";
            $mysql_password = "";
            $mysql_database = "web";
            //http://php.net/manual/en/pdo.construct.php
            $this->_conn = new PDO(
                'mysql:host=' . $mysql_hostname . ';' . 'dbname=' . $mysql_database,

                $mysql_user,
                $mysql_password
            );
        }

        public function getConn(){
            return $this->_conn;
        }
        /*
        $bd = mysql_connect($mysql_hostname, $mysql_user, $mysql_password) or die("Can't connect to database");

        mysql_select_db($mysql_database, $bd) or die("Opps some thing went wrong");
        */
    }
?>

//any space or new lines here ( before the open PHP tag ) will prevent the redirect
ini_set('display_errors', 1); //turn on error reporting when developing
include("db.php");
session_start();

if($_SERVER["REQUEST_METHOD"] == "POST"){
    $DB = new DB();

    // username and password sent from Form
    $username = $_POST['username']; 
    $password = $_POST['password']; 
    $password = md5($password); // Encrypted Password - update this as others have stated
    $sql="SELECT id FROM admin WHERE username = ? and passcode = ?";
    //http://php.net/manual/en/pdo.prepare.php
    $stmt = $DB->getConn()->prepare($sql);

    //http://php.net/manual/en/pdostatement.execute.php
    $stmt->execute( array( $username, $password ) );
    //http://php.net/manual/en/pdostatement.fetchall.php
    $results = $stmt->fetchAll(PDO::FETCH_ASSOC);

    // If result matched $username and $password, table row must be 1 row
    if(count( $results ) ){
        //session_register("username"); this is deprecated 
        $_SESSION['login_user']=$username;


        header("location: welcome.php");
        exit(); //add exit here
    }else {
        $error="Your Login Name or Password is invalid";
    }
}
?>

UserName :


Password :

for the redirect issue.
Don't use session_register();
How to fix the session_register() deprecated issue?