Are you planning on developing this primarily to sell to other people? If so, then I'd recommend selling it as a service rather than a product - not least because then the language its developed in is less of an issue.
Speaking as someone who has spent a lot of time supporting both Java and PHP apps, I do not understand why Java has so much more cachet in Enterprise circles. Especially when there are high profile sites (e.g. Facebook, sugarCRM) built in PHP.
"There are already a lot of open source issue trackers" - yes, but most of them are pretty awful.
my first instinct would be to build it using the Symfony PHP framework
Really? Sounds like you are trying to find something to write using Symfony rather than starting from the point of creating a great support tool, or a great business model.
It is a crowded market place - you need to think about what makes your idea better/more attractive than the many free and commercial packages.