Useful resources regarding the subject matter are as follow:

1. Starting Point: https://wiki.idempiere.org/en/Web_Services_First_Steps
2. Web Service Security: https://wiki.idempiere.org/en/Web_Services_Security
3. More Info: https://wiki.idempiere.org/en/Web_services
4. Access APIs: http://localhost:8080/ADInterface/services
5. To learn about CURL: https://phpenthusiast.com/blog/five-php-curl-examples
6. To Process Response: https://stackoverflow.com/questions/35581239/returning-a-xml-response-with-curl/35581517
7. To Beautify Response: https://www.semicolonworld.com/php/tutorial/convert-array-to-xml-php

Scenario:

Integrate iDempiere with PHP application and get all Business Partners using SOAP API.

Solution:

1. First of all, we have to setup web service cofigurations on iDempiere using Web Service Security window. Please follow the link # 1, mentioned above to learn how to do that.

2. After that write PHP Code and use CURL to trigger the API object queryData(). Here is the code: