You may use any of the following files (PHP or Python) to test the webhook notification system.
To use this, copy this file to the server where you're testing your webhook receiver. Change the output file name from /tmp/webhook_data.txt
to any other file name that your webserver can write to and that you can read.
Now, copy the link to the script and paste it under the Webhook URL
field of your Instamojo offer's edit page. Now, make a purchase of this offer. After the purchase is done, open the output text file. You should see the webhook data in this file.
You may modify the file according to your needs.