[php] jQueryAJAX - mysqli - utf8 - return json from query

jquery-php-mysqli.php

//index.html (jQuery)
    $.ajax({
        type : "POST",
        url : "getResults.php",
        data : "ID=" + "160",
        datatype : "json",
        success : function(result) {
            console.log(result);
        },
            error : function(msg) {
                console.log("error",msg);
            }
    });

//getResults.php
<?php
    try {
        include ('config.php');

        $db = connect();

        $stmt = $db -> prepare('SELECT * FROM test where test2_id = ? group by test4_id,test5_id');

        $p1 = "151";

        $stmt -> bind_param('s', $p1);  //$_POST['ID']);

        $stmt -> execute();

        $result = $stmt -> get_result();

        $returnVAR = array();

        //MYSQLI_NUM = Array items will use a numerical index key.
        //MYSQLI_ASSOC = Array items will use the column name as an index key.
        //MYSQLI_BOTH = [default] Array items will be duplicated, with one having a numerical index key and one having the column name as an index key.
        while ($row = mysqli_fetch_array($result, MYSQLI_ASSOC)) {
            $returnVAR[] = $row;
        }

        //unicode
        header("Content-Type: application/json", true);

        echo json_encode($returnVAR);

    } catch (exception $e) {
        echo json_encode(null);
    }
?>

//config.php
<?php

    function connect() {
        $mysql_hostname = "localhost"; 
        $mysql_user = "coin";
        $mysql_password = "P8";
        $mysql_database = "c3in";

        //setup a connection with mySQL
        $mysqli = new mysqli($mysql_hostname, $mysql_user, $mysql_password,$mysql_database);

        /* check connection */
        if (mysqli_connect_errno()) {
            printf("Connect failed: %s\n", mysqli_connect_error());
            exit();
        }

        //enable utf8!
        $mysqli -> query("SET character_set_results = 'utf8', character_set_client = 'utf8', character_set_connection = 'utf8', character_set_database = 'utf8', character_set_server = 'utf8'");

        return $mysqli;
    }

?>