auto generated number in php

I want to auto generate the id number. I write this code. But it's not work. I don't make primary key for id in database.

function getData()
{
  $data = array();

  $data[1] = $_POST['name'];
  $data[2] = $_POST['fname'];
  $data[3] = $_POST['address'];
  $data[4] = $_POST['phone'];
  $data[5] = $_POST['class'];
  $data[6] = $_POST['qualification'];
  $data[7] = $_POST['branch'];
  $data[8] = $_POST['rollno'];
  $data[9] = $_POST['gender'];
  $data[10] = $_POST['birth'];
  return $data;
}

//insert
if (isset($_POST['insert'])) {
  $info = getData();
  $insert_query = "INSERT INTO `smash`(`name`, `fname`, `address`, `phone`,`class`,`qualification`,`branch`,`rollno`,`gender`,`birth`) VALUES ('$info[1]','$info[2]','$info[3]','$info[4]','$info[5]','$info[6]','$info[7]','$info[8]','$info[9]','$info[10]')";
  try {
    $insert_result = mysqli_query($conn, $insert_query);
    if ($insert_result) {
      if (mysqli_affected_rows($conn) > 0) {
        echo ("data inserted successfully");
      } else {
        echo ("data are not inserted");
      }
    }
  } catch (Exception $ex) {
    echo ("error inserted" . $ex->getMessage());
  }
}