add_user.php

<?php
require "init.php";

    $username = $_POST["USERNAME"];
    $password = $_POST["PASSWORD"];
    $name = $_POST["NAME"];
    $surname = $_POST["SURNAME"];
    $login_state = $_POST["LOGIN_STATE"];
    $last_category = $_POST["LAST_CATEGORY"];
    $f_lang = $_POST["F_LANG"];
    $s_lang = $_POST["S_LANG"];


    $sqli = "INSERT INTO `user_info`(`ID`, `USERNAME`, `PASSWORD`, `NAME`, `SURNAME`, `LOGIN_STATE`, `LAST_CATEGORY`, `F_LANG`, `S_LANG`) VALUES (null,'$username', '$password', '$name', '$surname','$login_state','$last_category','$f_lang','$s_lang')";


    if(mysqli_query($con, $sqli)){
        echo "<br><h3>One Row inserted...</h3>";
    }else{
        echo "Error in insertion..." . mysqli_error($con);
    }

?>