<?php
        $db_username = $_POST['SUusername'];
        $db_password = $_POST['SUpassword'];

        $servername = "localhost";
        $username = "root";
        $password = "taddei123";
        $dbname = "tie1";


        $db = new mysqli($servername, $username, $password, $dbname);
        /* check connection */
        if ($db->connect_errno) {
            printf("Connect failed: %s\n", $db->connect_error);
            exit();
        }

        $db_user_query = $db -> real_query ("SELECT user_name
                                             FROM login_admin
                                             WHERE user_name = $db_username
                                            ");

        if ($db_user_query  == false){
            echo 'user ne postoji';
        }elseif($db_user_query == true){
            echo 'user postoji';
        }

    ?>