adding a paypal ipn redirect when selecting paypal on my websites drop down options

Question

php newbie here. I am looking to integrate a paypal ipn redirect upon the user selecting paypal from the drop down options on my processing.php page, any help will be much appreciated as struggling abit and trying to learn more about this process, if anybody could walk me through this that would be great! many thanks

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

class Processing extends CI_Controller
{


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

        if ($this->session->userdata("userID") == "") {
            $this->session->set_flashdata("processloginredirect", 1);

            redirect(base_url());
            exit;
        }


    }


    public function index()
    {
        $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

        $charactersLength = strlen($characters);

        $randomString = '';

        for ($i = 0; $i < 14; $i++) {

            $randomString .= $characters[rand(0, $charactersLength - 1)];

        }
        $this->session->set_userdata("assigntransactionID", $randomString);


        $data['allcurrency'] = $this->db->query("select * from allcurrency where isactive = 1")->result_array();


        $data['allsendingcurrency'] = $this->db->query("select *,c.id as currencyID from allcurrency as c inner join sending_accounts as sa on (c.id = sa.currencyID) where c.isactive = 1 order by sa.ordernumber")->result_array();

        $data['allreceivecurrency'] = $this->db->query("select *,c.id as currencyID from allcurrency as c inner join receive_accounts as ra on (c.id = ra.currencyID) where c.isactive = 1 order by ra.ordernumber")->result_array();


        if ($this->session->userdata("youhave_currency")) {

            $youget_currency_data = $this->db->query("select * from allcurrency where id = " . $this->session->userdata("youget_currency") . "")->row_array();

            $youhave_currency_data = $this->db->query("select * from allcurrency where id = " . $this->session->userdata("youhave_currency") . "")->row_array();


            $site_config = $this->db->query("select * from site_config")->row_array();


            $data['userwalletinfo'] = $this->db->query("SELECT * FROM allcurrency AS c INNER JOIN players_wallet AS pw ON (c.id = pw.currencyID) WHERE 
     pw.playerID = " . $this->session->userdata("userID") . " AND c.symbol = '" . $youhave_currency_data['symbol'] . "'")->row_array();

            $admin_account_detail = $this->db->query("select * from allcurrency as c inner join receive_accounts as ac on (c.id = ac.currencyID) where c.id = '" . $youhave_currency_data['id'] . "'")->row_array();


            if ($admin_account_detail['iscryptocurrency'] == 0) {


                $accountinfo = '';
                $accountforcopy = '';
                if ($admin_account_detail['accountemail'] || $admin_account_detail['accountwalletkey']) {
                    if ($admin_account_detail['accountwalletkey']) {
                        $accountinfo .= "Account Address : " . $admin_account_detail['accountwalletkey'];
                        $accountforcopy = $admin_account_detail['accountwalletkey'];
                    } else {
                        $accountinfo .= "Account Address : " . $admin_account_detail['accountemail'];
                        $accountforcopy = $admin_account_detail['accountemail'];
                    }
                }
                if ($admin_account_detail['mobilenumber']) {
                    $accountinfo .= "<br>Mobile Number : " . $admin_account_detail['mobilenumber'];
                }
                if ($admin_account_detail['idcardnumber']) {
                    $accountinfo .= "<br>ID Card Number : " . $admin_account_detail['idcardnumber'];
                }


                $adminwallet_data = $accountinfo;


            } else {
                $adminwallet_data = $admin_account_detail['accountwalletkey'];
            }


            $data['adminwallet'] = $adminwallet_data;//$admin_account_detail['symbol_title']." Address : ".$admin_account_detail['accountwalletkey'];    
            $data['adminwallet_forcopy'] = $accountforcopy;

            $data['sendcurrencynumber_step4'] = $this->session->userdata("youhave") . ' ' . $youhave_currency_data['symbol_title'];


        } else {
            $data['userwalletinfo'] = array();
            $data['adminwallet'] = '';
            $data['sendcurrencynumber_step4'] = '';

        }


        if ($this->session->userdata("youhave_currency")) {
            $data['youhave'] = $this->session->userdata("youhave");
            $data['youhave_currency'] = $this->session->userdata("youhave_currency");
            $data['youget_currency'] = $this->session->userdata("youget_currency");
            $data['youget'] = number_format($this->session->userdata("youget"), 10);
        } else {

            $json = file_get_contents("https://min-api.cryptocompare.com/data/price?fsym=BTC&tsyms=ETH");
            $asdasdasdsa = json_decode($json);

            $get_currency_amount = $asdasdasdsa->ETH * 1;

            $depositefee_chrge = $site_config['depositefee'] * $get_currency_amount / 100;

            $get_currency_amount = $get_currency_amount - $depositefee_chrge;

            $this->session->set_userdata("youhave", 1);
            $this->session->set_userdata("youhave_currency", 1);
            $this->session->set_userdata("youget", number_format($get_currency_amount, 10));
            $this->session->set_userdata("youget_currency", 2);

            $data['youhave'] = $this->session->userdata("youhave");
            $data['youhave_currency'] = $this->session->userdata("youhave_currency");
            $data['youget_currency'] = $this->session->userdata("youget_currency");
            $data['youget'] = $this->session->userdata("youget");


            $admin_account_detail = $this->db->query("select * from allcurrency as c inner join receive_accounts as ac on (c.id = ac.currencyID) where c.symbol = 'BTC'")->row_array();

            $data['adminwallet'] = $admin_account_detail['symbol_title'] . " Address : " . $admin_account_detail['accountwalletkey'];     //$admin_account_detail['accountwalletkey'];   

            $data['sendcurrencynumber_step4'] = $this->session->userdata("youhave") . ' ' . $admin_account_detail['symbol_title'];