I'm Working in a french bank, boursorama.com. We have 3 similar banks in spain, UK & Deustchland. We use PHP for pretty everything except heavy load application (C & C++ instead). We have some Java proxy, provided by partners that don't have any other solutions.
I think PHP is not used by Bank Companies mostly because their technology stack is older than PHP5 himself. Before PHP5, you can't except getting out serious object oriented features.
From my experience, Bank don't have a single technology stack. They works with external partners through services that don't need to fit in your technologies (SOAP, REST, ...). So, all-in-all, you partners don't need to be involved in your tech choices.