Fatal error:uncaught error:call to undefined mysql_real_escape_string()

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PHP

MySQL

I am new to PHP and my sql. I created i register form for users to registee but i get this error

Fatal error:uncaught Error:Call to undefined mysql_real_escape_string()

. here is my code.

<?php
 session_start();
 
// connect to databse
$db = mysqli_connect("localhost", "dbnameuser","password","dbname");

if (isset($_POST['register_btn'])){
	session_start();
	$username = mysql_real_escape_string($_POST['username']);
	$email = mysql_real_escape_string($_POST['email']);
	$password = mysql_real_escape_string($_POST['password']);
	$password2 = mysql_real_escape_string($_POST['password2']);
	
	if ($password == $password2){
		//create user
		$password =md5($password);// hash password before storing for security purposes
		$sql = "INSERT INTO users(username,email,password) VALUES('$username','$email,'$password')";
		mysqli_query($db, $sql);
		$_SESSION['message'] = "You are now logged in";
		$_SESSION['username'] = $username;
		header("location: home.php"); //redirect to home page
		
	}else{
		$_SESSION['message'] = "The two passwords do not match";
	}
	}

?>

What I have tried:

I searched online and this site seems to have more info on the fix e.g

mysql_real_escape_string() extension 

is removed, i dont know what exact syntax to enter for the

MySQLi or PDO_MySQL extension

they sugggest to use .