How to integrated payment gateway in my form - php

Question

Really I am confused how to start..
I have a submit form in my webpage. which contains name, address, email etc. along with Amount To Be Paid (fetched from mysql database).
Now I want user to redirect to payment gateway with amount ((which is not fixed, but changing) and ONLY after successful payment, the form should be get submitted and data should be get added to mysql database.
I have downloaded Instamojo payment gateway api details and I already have active account on Instamojo.
My current code for Submit Form is as follows...
<?php
include("db.php");
if($_POST['action']=="registration"){
$all_columns[]="fname";
$all_columns[]="mname";
$all_columns[]="lname";
$all_columns[]="email";
$all_columns[]="amount_paid";
$all_values[]=addslashes($_POST["fname"]);
$all_values[]=addslashes($_POST["mname"]);
$all_values[]=addslashes($_POST["lname"]);
$all_values[]=addslashes($_POST["email"]);
$all_values[]=addslashes($_POST["amount_paid"]);
$qry=$estambha->webdreaminsert("registration",$all_columns,$all_values,'','','');
$_SESSION["Register_id"]="suceess";
header("location:registration-confirm.php");
}
?>
And HTML Form code is as follows :
<form name="registration_form" id="registration_form" action="" method="post" enctype="multipart/form-data">
<input type="hidden" name="action" value="registration" />
Name :<br />
<label for="fname">First Name :</label> <input name="fname" type="text" id="fname" required/>
<label for="mname">Middle Name :</label> <input name="mname" type="text" id="mname" required/>
<label for="lname">Last Name :</label> <input name="lname" type="text" id="lname" required/>
<br />
<br />
E-mail :<br />
<input name="email" type="text" id="email" required/>
<br />
<br />
Registration Amount:<br />
<?
$paymentdata=mysql_query("select * from event_calendar where branch='Nk' group by part DESC limit 1");
$numrows1=mysql_num_rows($paymentdata);
while($amount=mysql_fetch_array($paymentdata)){?>
<input type="text" id="amount_paid" name="amount_paid" value="<?=$amount['fees_for_online']?>" readonly="readonly" />
<? }?>
<input name="formsubmit" type="submit" id="inputsubmit1" value="Register Me" />
Currently After Form Submission, Data get added to mysql database. But I want To Redirect user to Instamojo Payment Gateway Page with Amount from readonly field and after successful payment, user should redirect to registration-confirm.php page and during this data should be get added to mysql.
I am not getting any idea how to do this. I went through Instamojo documents too...Till searching for solution...

Answer 1

ok, let me start with the simple basic process which is followed by all payment integration modules (Process is based on authorized and paypal payment gateway).
1) Collect the data required you payment like user information and payment details like amount and quantity.
2) Payment gateway provides a form which need to be submitted on their sandbox account (URL given in documentation). you can submit the form either CURL or by simple jquery form submit. If you are using jQuery then form must be generated on runtime.
3) In the form provided by the payment gateway, there are two URL success url and the fail url. When the user transaction is complete depends on result, payment gateway redirect to your site on either URL.
4) Get the responce return by payment gateway on relevant success or fail page and process the data.