Does Deutsche Bank use php? Does HSBC use fucking MYSQL???
I can assure you that there are many MySQL instances inside any bank you could point your finger at. Several of them business critical for their respective environments.
It is guaranteed to be a lot of PHP too, just not customer facing. Banks are big things with lots of IT.
Hmmmm.... not really ...not in the golden circle of large blue chip banks
A bank will have a core apps platform maybe 30-100 apps and I gurantee there wont be mysql & php kicking around