This Is A Secure Way To Use mysqli::prepare
--------------------------------------------------------
<?php
function secured_signup($username,$password)
{    
$connection = new mysqli($dbhost,$dbusername,$dbpassword,$dbname);    
if ($connection->connect_error) 
die("Secured");
    
$prepared = $connection->prepare("INSERT INTO `users` ( `username` , `password`  ) VALUES ( ? , ? ) ; ");
if($prepared==false)
die("Secured");
    
$result=$prepared->bind_param("ss",$username,$password);
if($result==false)
die("Secured");
    
$result=$prepared->execute();    
if($result==false)
die("Secured");
        
$prepared->close();
$connection->close();    
}
/*
$dbhost ---> DataBase IP Address
$dbusername ---> DataBase Username
$dbpassword ---> DataBase Password
$dbname ---> DataBase Name
*/
?>