Ajax: forced utf-8 encoding

I have a webpage in UTF-8 defined in my head

<meta charset="utf-8" />

Then I have an external .js file containing the following code:

function ajax_db(){
    $(document).ready(function(){
//some variables defined

        $.ajax({
            type:"POST",
            url:"db.php",
            data:"u="+uname+"&p="+pass+"&m=1",
            dataType:"text",
            contentType: "application/x-www-form-urlencoded;charset=utf-8",
            success: function(reply){
                $("#regstat").html("<p class='status'>"+reply+"</p>");
            }
        });
    });
}

As you can see, the request is sent to db.php, and if m is set to 1, and not all fields are filled, it ends here:

$mode = htmlspecialchars($_POST['m']);

//Mode: 1 - register, 0 - log in, 2 - log out

if ($mode == 1){

if (empty($_POST['u']) || empty($_POST['p']) || empty($_POST['email'])){
    $reply = "";
    echo utf8_encode($reply);
    exit;
}
//more code
}

The problem is that it returns some mojibake instead of text. I tried to force utf-8 encoding, wherever I am, I entered db.php with the following lines:

mb_internal_encoding( 'UTF-8' );

header("Content-type: text/html; charset=utf-8");

No effect. In addition, if I delete the utf8_encode () function, the question mark fields are returned, which should indicate that utf-8 is not used.

Where can the problem arise?

Thank you for your time.