使用RSA-SHA256为PHP中的Oauth 1.0生成签名

How to generate signature using RSA-SHA256 for Oauth 1.0 Authentication API consumption? Is there any inbuilt function available for it in PHP. I want to generate oauth_signature for consuming /oauth/request_token end-point on https://www.interactivebrokers.com/webtradingapi/

I am not getting the way to generate the signature. Can anyone also provide a link explaining the RSA-SHA256 algorithm. Thank You in Advance.

RSA-SHA256