Hi
You can write a new payment plugin and installed it via Payment Form -> Payment plugins . There are serveral payment plugins available in the package, the code is clear written, so I think you can write the plugin yourself easily (you said you are an experienced PHP developer) .
In your case, the payment plugin is a credit card payment plugin, so I suggest you have a look at the authorize.net payment plugin in the package . There are only two files : os_authnet.php and os_authnet.xml . It is easy to understand .
You can go ahead to implement the plugin. If you have any difficulty or have any questions, I am available for support !
Regards,
Tuan