WHMCS Gateway Callback Setup

[technosect]

Hey there,

I am creating my own Payment Gateway module.

On the invoice page, I have created an input field to take a value. User putting a value on that field and clicks Pay button. And I am sending the value to the pay_process.php page in the pay_process.php page it is checking the value from the database if the value is valid or not if the value is valid then the invoice will be paid.

Now the issue is that after validating the value with database how should I use the callback file to mark the invoice as paid.

What will be the code for the callback file redirection and how the invoice will mark as paid?    

Here is the code: 

pay_process.ph

<?php
        session_start();
        $usr_name = require 'getLogin.php';
        if ($usr_name != false) {
            $_SESSION['user'] = $usr_name;
        }
        
        $username = $_SESSION['user'];
        $hostname = 'localhost';
        $db = 'db_name';
        $duser = 'db_user';
        $pass = 'db_pass';
        $table = 'dn_tokens';
        $invoice_id =$_POST['invoice_id'];
        $description = $_POST['description'];
        $amount =  $_POST['amount'];
        $currency = $_POST['currency'];
        $ress = preg_replace("/[+]/", "", $description);
        $token = $_POST['token'];
        $_SESSION['token_val'] = $token;
        
        //Database Connect
        $conn = mysqli_connect($hostname,$duser,$pass,$db);
        
        // Check connection
        if (mysqli_connect_errno()){
            echo "Failed to connect to MySQL: " . mysqli_connect_error();
        }
        
        //Cross Verification
        if(isset($_GET['token'])){
             $_SESSION['token_val'] = $_GET['token'];
            $sql2 = "SELECT owner, extension, token, status FROM $table WHERE token = '".$_SESSION['token_val']."'";
            $res2 = mysqli_query($conn, $sql2);
            $row2 = mysqli_fetch_array($res2,MYSQLI_ASSOC);
            $owner = $row2['owner'];
            if($_SESSION['user'] != '' && $_SESSION['user']==$owner){
                $msg = '<b>Sucessfully Verified!</b';
                $msg1 = 'verified'; 
                function callback(){
                    
                }
            }
            else{
                $msg = 'Token Owner Mismatch!';
            }
        }
        else{
            //Verify Token
            $sql = "SELECT owner, extension, token, status FROM $table WHERE token = '".$_SESSION['token_val']."'"; 
            $res = mysqli_query($conn, $sql);
            
            if($res && mysqli_num_rows($res) > 0){
                $row=mysqli_fetch_array($res,MYSQLI_ASSOC);
                $owner = $row['owner'];
                if($row['status']=='active'){
                    $msg = 'This token Belongs to: '.'<b>'.$row['owner'].'</b>';
                    if($_SESSION['user'] != '' && $_SESSION['user']==$owner){
                        $msg = 'Owner Verified!';
                        $msg1 = 'verified';
                    }
                    else{
                       $msg1 = 'verify';
                       $verify = '<br/><br/><center><a href=https://steemconnect.com/oauth2/authorize?client_id=dnom.app&redirect_uri=https://dnom.io/callback.php&scope=login%2Cvote%2Ccomment%2Ccomment_options%2Ccustom_json&state=/pay_process.php?token='.$_SESSION['token_val'].'>Login to Verify Payments</a></center>';
                    }
                      
                }
                else{
                    $msg =  'Token Alreday Used!';
                }
            }
            else{
                    $msg = 'Invalid Token Number!';
            }
        }
?>

 

Someone, please help me.

Thanks in advance!