title: Why checksum is used by payment gateways?
body: <p>I have been intergrating a payment gateway with my site.In their documentation they have specified to make a checksum</p>
  
  <pre><code>$checksum = $merchantid."|".$amount."|".$customerReferenceNo;   
  $checksum = hash('sha256', $checksum);  
  </code></pre>
  
  <p>as in their PHP documentation.</p>
  
  <p>I am wondering why this is used and how they will make use of the checksum I sent them.
  Thoughts?</p>