How can I use transactions with these queries?

Hey guys;

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How can I use transactions here?

Would it be right if I simply add $this->db->trans_start(); and $this->db->trans_end(); to the beginning and end of index method?

Thanks

PHP Code:

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Data_receiver extends CI_Controller {

 public function __construct()
 {
 parent::__construct();
 }

 public function index()
 {
 //Validate Account Owner
 $sql = "SELECT * FROM launch_owners WHERE id = ?";
 $query = $this->db->query($sql, array($_POST['user_id']));
 if($query->num_rows()!=1)
 {
 echo "Invalid Request";exit();
 }
 else
 {
 $results = $query->result_array()[0]; 
 //More Validate Account Owner
 if($results['status']!='active' || $results['secret_key']!=$_POST['secret_key'])
 {
 echo "Inactive Account Or Invalid Secret";exit();
 }
 else 
 {
 //Validate Launch
 $sql = "SELECT * FROM launch_launches WHERE id = ? AND launch_type = ?";
 $query = $this->db->query($sql, array($_POST['launch_id'], 'evergreen'));
 if($query->num_rows()==1)
 {
 //Does Prospect exist under this user?
 $sql = "SELECT * FROM launch_prospects WHERE email = ? AND owner_id = ?";
 $query = $this->db->query($sql, array($_POST['prospect_email'], $_POST['user_id']));
 if($query->num_rows()==1)
 {
 $prospect_id = $query->result_array()[0]['id'];
 }
 else 
 {
 $data = array(
        'email' => $_POST['prospect_email'],
        'owner_id' => $_POST['user_id'],
 );
 $this->db->insert('launch_prospects', $data);
 $prospect_id = $this->db->insert_id();
 }
 //Delete prospect from this launch if already exist in the seqeunce
 $this->db->delete('launch_launch_prospect', array('launch_id' => $_POST['launch_id'], 'prospect_id'=>$prospect_id)); 
 //Add Prospect To Launch
 $data = array(
        'launch_id' => $_POST['launch_id'],
        'prospect_id' => $prospect_id,
        'time_added' => time(),
        'date_added' => date('Y-m-d')
 );
 $this->db->insert('launch_launch_prospect', $data);

 //Insert the same into stats table
 $data = array(
        'launch_id' => $_POST['launch_id'],
        'prospect_id' => $prospect_id,
        'time_added' => time(),
        'date_added' => date('Y-m-d')
 );
 $this->db->insert('launch_launch_prospect_history', $data);
 echo "Added Successfully";
 }
 else 
 {
 echo "Invalid launch";exit();
 } 
 } 
 } 





 }
}