RPC is a smart method to interaction between two different process of a same/different system in a secured way. WSDL is a w3 standard for information sharing over internet and soap is also a w3 standard. WSDL and SOAP uses XML as standard for transferring data which helps to communicate two different type of system (example. windows to unix) or two different software of different platforms.
For securing the data manipulated by the clients Banks uses some APIs which uses WSDL and SOAP standards to communicate over http. cURL helps to PHP programmers to make a http request over internet. But working with PHP for this kind of job is not so easy. For other APIs it is not so complicated, but easier. Banks uses more security so programmers should do it carefully to integrate the system with those APIs.
Usually Banks uses two different security system together. They allows only some predefined ip address with defined SSL certificates. Most of them uses pkcs12 formatted SSL, but PHP doesn't like this. For using this format to communicate. Programmers usually breaks this SSL into three different parts of PEM formatted SSL. Then it is verified by cURL codes and then the response could be found from the server. It's complicated.