utf 8 - php encoding of posted data changed

I post data from client to php, here is how my post data
seems in client:
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I click
add and post this string to php with ajax call here how posted data seem in ajax
parameter;

//username ="Uğur Akkaynak" so
            its ok.
 var params = {
 action: "saveAdObjects",

            username_: username,
 }
 $.ajax({
 url:
            "../../Controller/ActiveDirectoryController.php5",
 type: "POST",

            async:false,
 dataType: "json",
 data: params,
 success:
            function (result)...

In php file
I check the posted variable:

var user =
            $_POST["username"]; //UÄŸur Akkaynak
            wtf!

so problem is clear, strings
broken in php, I know I need change encoding in php file and tried
these:

header('Content-type: text/plain;
            charset=utf-8');
utf8_encode($_POST["username"]);// Uğur
            Akkaynak
mb_convert_encoding ($_POST["username"],'utf-8'); // UÄŸur
            Akkaynak

What should I do get
$_POST["username"] as 'Uğur Akkaynak' like posted from client