LOGIN
$username=$_POST['username'];
$password=$_POST['password'];

$sql = "SELECT * from profile";
$result = mysqli_query($conn,$sql);

if ($result->num_rows>0) {
    while ($row=$result->fetch_assoc()) {
        if ($row["username"]==$username && $row["password"]==$password) {
            echo "Logova si se,Brao";
            # code...

REGISTER

$conn = mysqli_connect($servername,$username,$password);
mysqli_select_db($conn,$dbname);

    $username=$_POST['username'];
    $password=$_POST['password'];
    $name =$_POST['name'];
    $last_name=$_POST['last_name'];
    $birthday=$_POST['birthday'];

    $upit="INSERT INTO profile(username,password,name,last_name,birthday)"."VALUES('$username','$password','$name','$last_name','$birthday')";
    mysqli_query($conn,$upit);

UPDATE

    $id = $_POST['id'];
    $username = $_POST['username'];
    $password = $_POST['password'];


    $sql = "UPDATE licni_podaci SET username = '$username', password = '$password' WHERE id = $id";
    mysqli_query($conn,$sql) or die("Greska u upitu");

DELETE

sql = "DELETE FROM licni_podaci WHERE id = $id";
    mysqli_query($conn,$sql) or die("Greska u upitu");
    echo "Uspjesno ste obrisali";
    exit;