http://developer.pesapal.com/how-to-integrate/api-reference#PostPesapalDirectOrderV4

Here is a link to integrate pesalpal payment gateway.

PesaPal uses the OAuth protocol to authenticate API requests. The request is signed using the Consumer Secret you received at registration.

You may download OAuth libraries for PHP and .Net from our downloads section. For a list of other OAuth libaries, please visit: http://oauth.net/code/

How can I generate correct oauth_signature?

More information on OAuth can be found at http://oauth.net/ or in the Beginner's Guide to OAuth from Hueniverse http://hueniverse.com/oauth/