PHP Form Not Inserting

PHP Snippet 1:

<form>
  <input type="text" name="sendData[fieldname]">
  <input type="submit">
</form>

PHP Snippet 2:

if(isset($_POST['sendData'])) {
    $fieldname = $_POST['sendData']['fieldname'];
}

PHP Snippet 3:

$run_query = mysqli_query($conn,$insert_product)