you should use this code
$yourConnection->set_charset("utf8");
for example
function insertUser($inUN,$inP,$inFN,$inLN,$inE,$inD)
    {   
        $servername="localhost";
        $username="mnr";
        $password="milad1373";
        $databacename="mnr01";
        $conn = new mysqli($servername, $username, $password, $databacename);
        if ($conn->connect_error) {
            die("Connection failed: " . $conn->connect_error);
        } 
        $conn->set_charset("utf8");
        $sql = "INSERT INTO chuser (username, passWord, fname,lname,email,date)
        VALUES ('$inUN', '$inP','$inFN','$inLN', '$inE','$inD')";
        if ($conn->query($sql) === TRUE) {
            echo "New record created successfully";
        } else {
            echo "Error: " . $sql . "<br>" . $conn->error;
        }
        $conn->close();
    }
i had this problem but it resolved by this code
and for more information go to this link